Further comments for the sake of completeness..
As pointed out above, the discussion is somewhat academic (although conceptually, still very important to properly understand how the rules work) if there is no valid protest. If the incident was observed by the race committee or protest committee (or even the technical committee), they could potentially lodge a protest so long as the validity requirements were met. In practice, this is more likely to happen if they believe there is a possibility that a boat has broken RRS 2.
A boat knowingly touching a mark and not taking a penalty is the classic violation of the principles of sportsmanship. However, if the leeward boat was deemed by the protest committee to be exonerated under RRS 43 then it obviously follows they have not broken RRS 2 as they were not required to take a penalty or lodge a protest.
If the protest committee found that the leeward boat was not exonerated under RRS 43 then she would be disqualified for breaking RRS 31. It does not necessarily follow however that the boat has also broken RRS 2. If the leeward boat genuinely believed she could have been exonerated by the windward boat's actions, she has not breached RRS 2 even if her belief was mistaken.
How do you establish whether the leeward boat had a genuine belief they were exonerated? It's obviously not possible to get inside the skipper's head, so the protest committee would needs to find relevant facts in order to make an inference. For example, establishing what the separation between the boats was. Other (more circumstantial) factors could possibly include the skipper's racing experience and knowledge of the rules. Notwithstanding the above comments, the fact the leeward boat didn't protest the windward boat could be a relevant factor, albeit not a conclusive fact. It's important to note that under RRS 2 a boat can only be penalised if it is 'clearly established' that the principles of sportsmanship have been violated, so the boat is entitled to the benefit of the doubt to some extent.
TLDR version - as always with any good question, the answer all depends on the facts!