Yes
There is no rule that requires a boat to sail a proper course. The leeward boat could sail on past a Windward mark and not bear away, pinning the Windward boat. 
 See case 9 world sailing

Unless 17 applies, or the outside boat happens to have mark room (which could happen).

It would also depend on how the leeward boat became overlapped - RRS 17 might apply and give her a proper course limitation.

Or if the proper course to the next mark requires a gybe.  18.4

That depends.  If the leeward boat tacked onto the layline inside the zone, then she's subject to rule 18.3, which says she has to give the other boat mark-room.  She therefore not only has to turn down, she has to give the other boat room to turn down as well.  If she obtained her overlap from astern, she is subject to rule 17 and cannot sail above her proper course, which is usually to turn onto the next leg.

On the other hand, the leeward boat has right of way, so if she did not tack in the zone to round the mark she can sail straight.  In fact, she can sail any course she likes, but if she luffs, she has to give the windward boat room to keep clear.  Whether she has to give the windward boat room to keep clear when she eventually bears off after sailing straight is something of an open issue.  If rule 18 still applies to the boats and mark-room is still defined, then she can bear off any way she likes, because she is sailing within the mark-room to which she is entitled and will be exonerated under rule 43.  After the other boat has given her the mark-room she is entitled to, then when she turns down she has to give the windward boat room to keep clear. 

Mark-Room Room for a boat to leave a mark on the required side. Also,
(a)  room to sail to the mark when her proper course is to sail close to it, and

(b) room to round or pass the mark as necessary to sail the course without touching the mark. 

18.1 When Rule 18 Applies
Rule 18 applies between boats when they are required to leave a mark on the same side and at least one of them is in the zone.
...
Rule 18 no longer applies between boats when mark-room has been given.

43 EXONERATION
43.1 (b) When a boat is sailing within the room or mark-room to which she is entitled and, as a consequence of an incident with a boat required to give her that room or mark-room, she breaks a rule of Section A of Part 2, rule 15, 16, or 31, she is exonerated for her breach. 
 

 Rob,

If the leeward boat tacked inside the zone, 
1. she would not be required to give the windward boat mark-room (the windward boat would not have an inside overlap).
2. she could sail on past the mark as long as she did not cause the windward boat to sail above close-hauled (if the windward boat had been on starboard tack since entering the zone). 

Hi guys, nice to join again.
i'd like to add: why the leeward boat did not bear away after rounding the windward mark, causing the windward boat to lose time and water?
it's an hindering. So i'd check wether the hindering has been made for her own scoring in the race or series, or if there is a sort of team racing going on.
As per RRS 18,3 i agree with Murray. 

Hi Aldo 
What rule do you think the leeward boat broke by not bearing away?

If no 17, and it's proven a team racing, RRS 2. (for new guys, it means that she hinder that boat to make a favour to a third boat, like a pal of same club, or a relative, or because the hindered boat's skipper has an affair with his wife)
Well, if that boat's scoring has nothing to do with the scoring of the hindered boat, she has to explain to PC the reasons of that action. And squeezed her, at least to scare her, not to hinder again, unless for his own scoring benefit.

Aldo, re: ".., if that boat's scoring has nothing to do with the scoring of the hindered boat, she has to explain to PC the reasons of that action."

I disagree with your reply above.

For the inside boat to be required to turn-down after passing the windward mark, a rule referencing 'proper course' would have to apply in the situation.   RRS 18.3 shouldn't be an issue as mark-room in 18.3 only obligates the outside boat to give the inside boat mark-room in specific circumstances.  If RRS 18.3 applies, 18.3's limitation on the inside boat is not to cause the outside-windward boat  ".. to sail above close-hauled to avoid contact" .. and says nothing the inside boat not sailing above her proper course.

What rule do you believe applies in this circumstance which references "proper course", "hinderance" or "interference" that would obligate inside to turn-down after rounding?

Hi Angelo
As said, no 18.3 involved.
Hindering not for own scoring breaks RRS 2, better explained in case 78

Aldo .. I get Case 78 .. but we are talking here about a leeward boat holding her course vs a windward boat after she passes the windward mark.  This a boat-on-boat situation where there are many clear reasons why someone would choose that, primarily that the leeward boat wants to end up being to windward with clear-air for downwind leg.

Assuming these 2 boats are in the same fleet and racing against each other, a RRS 2 complaint in this case would be pretty bizarre IMO.

It's like  suggesting that a leeward boat on the run has to justify heading-up a boat that is over-taking her to windward to protect from being rolled.

Wel, Angelo, you can assume they are racing one against the other. Still, i'd like do be sure of that.
The hindering is there, so she has to explain. Team racing happens.