g) Score the boat NSC   (Appendix A5.1)

And h) check sailing instructions, since, in the UK at least, this situation is often covered in club racing SIs. 

Jim, 
How do you cover this in the SI in UK?

Typically you'll see an SI on these lines:

"6.3 Incorrect Course. If a member of the Race Committee observes a boat sailing the wrong course and as a result breaks RRS 28.2 he may disqualify that boat without a hearing. This changes RRS 63.1." 

From https://www.stmawessailing.co.uk/downloads/2019JRW-SI.pdf

I offer no opinion on whether its a good idea or not, merely that something on those lines is not unusual hereabouts. Perhaps could be worded better, just the one that came first with a Web search. 

I guess the NSC provision in A5.1 renders that SI obsolete now.

I'd score NSC but I'd ensure that the finish recording sheets also showed the boats position and time of finishing (for handicap sailing) in case there was a successful appeal against my decision.   

Thank you all.   I should have read Appendix 5.1......

If a boat radioed in that he had not rounded a mark due to the actions of another boat would that be sufficient witnessing for a RC to score her NSC?

I think if I were on the RC I would want to be very confident that the boat had not subsequently rounded the mark and sailed the course correctly. It seems to me that a competitor might announce on the radio that someone had forced them the wrong side of the mark, but be less likely to comment that they had subsequently rounded it properly. I would also have grave doubts about whether overhearing something on the radio adequately constituted observing in the race area (61.1b). 

I once asked a similar question to a bunch of peers. 

The general concensus was that to be safe, that word 'witnessed ' ought to be taken pretty literally. 

In Paddy's case, then no. It would not be a safe NSC in my opinion.

Thanks Jim and Benjamin your comments are very helpful

Just a bit of context all boats are expected to have vhf turned on and tuned to the specified channel. All radio messages would be heard by all boats racing. 
Boat A radioed the RC saying that because boat B had hit the mark and rounded up, she was prevented from rounding the mark and that she intended to protest. 
The RC returned boat A’s radio message saying that this issue would be handled on shore. 
So the RC was fully aware of the situation not just overheated it.
Would this have any further bearing on my question. Should the RC investigate boat A. 
Boat A was scored in her finishing position with PHS applied. 

Paddy,

I'm afraid I think this is sounds like Boat A is 'challenging' the RC to deal with her problem, rather than dealing with the situation herself.  It smells of a Rule 2 breach in my opinion.

(In fact, I question why Boat A would claim she was unable to round the mark, simply because a boat had hit it and rounded up.  In most cases, there is nothing to prevent a Boat A rounding both the Boat B and the mark, even if that needs some extra maneouvring.)

If Boat A has admittedly not sailed the course, she should either take a penalty or if she feels she was compelled to do so by another boat's breach, SHE should seek the resolution according to the system.

It is not up to the RC to penalise her for a breach of Rule 28 they did not witness.

As it stands, Boat A should probably retire.  Additionally, it seems she has lost the option to protest Boat B.  In failing to retire here, Boat A has broken Rule 2, and possibly Rule 69 if it becomes clear she was gaming the system.

Thanks again Benjamin
You have made a good case. And I have learned that it is important that a breach must be actually witnessed/seen by RC  for them to proceed to any action. 

 Was there in fact a valid protest by boat A that led to a hearing? If so it seems to me everything connected to the incident would have been best dealt with there. In the UK we have RYA case 1982/10 which covers pretty much exactly this situation, and part of the result of the hearing would presumably be that boat A would be scored NSC.

 If there was no valid protest then I submit the question is whether it would be appropriate for the RC to initiate a protest against boat A for allegedly not sailing the course. It doesn't seem to me there is sufficient evidence to score NSC without a hearing. In this company I shall defer to wiser heads and not offer an opinion. 

FWIW here is the RYA case.

RYA 1982/10
Rule 28.2, Sailing the Course
Rule 62.1(d), Redress
A boat that has been forced the wrong side of a mark is
not exempted by any rule from sailing the course, nor is
redress normally available to her.
SUMMARY OF THE FACTS
At a mark, I was overlapped inside O before the zone was
reached, and was therefore entitled to mark-room under
the first sentence of rule 18.2(b). There was a collision
just before the mark, and I, having no room to pass
between O and the mark, left it to port, instead of to
starboard as required by sailing instructions. She did not
subsequently return and pass it on the correct side. She
protested O. The protest committee disqualified both
boats, O under rule 18.2(b) for not giving mark-room,
and I for failing to sail the course. It concluded that O’s
breach had been careless rather than deliberate. I
appealed.
DECISION
I’s appeal is dismissed.
There is no racing rule that exempts a boat from
complying with rule 28.2. Even had she returned,
unwound if necessary and then rounded on the correct
side, she would not have been entitled to redress for
places lost, since none of the grounds in rule 62.1 was
applicable. Rule 2 had not been broken, nor would a
hearing under rule 69 have been appropriate, so no
request for redress under rule 62.1(d) in particular could
have succeeded.

Jim no protest lodged. 
Have read the 1982/10
And yes can’t think of a scenario where you could miss a mark and not break 28.2. 
It becomes complicated in mixed fleet racing when some boats only have a fleeting knowledge of RRS 
Thanks for your input