Hmmmm....
If one of the requirements at the beginning of the skirmish is to "wound him twice", and he puts on the ring before the second wound hits, he still HAS NOT wounded Boromir twice.
It is clear that with this ring it says "if bearer IS ABOUT TO take a wound" you put the ring on. If the FP player responded in putting the ring on before the second wound, then he added a burden INSTEAD of wounding Boromir. Boromir was only wounded once. Therefore the skirmish can't start until he is again wounded, or three burdens are added. I'm not sure if at this point he would get the option to add three burdens, or, since you made the choice to wound him twice, he would continue TRYING to be wounded until he is corrupted (because at this point, he is adding burdens one at a time, and not "adding three burdens").
I wouldn't be so quick to say that Boromir isn't bugged here. I used Faramir with this ring once, trying to wound him to make a minion roaming. I "wounded" him, but then used the ring to turn the wound into adding a burden. However, since I never actually wounded him, the minion did not become roaming. This is correct behavior. Faramir was never actually wounded, therefore the minion did not become roaming. For this reason, I believe it IS possible that Boromir is bugged. Because, I submit that the correct behavior would be, same as Faramir, like this: Boromir was never wounded twice, therefore the skirmish does not start.
In this case starting the skirmish is a required action, so the conditions MUST be continue to TRY to be met until they are met, or either Boromir is killed or corrupted.
I understand what MarcinS meant by: "because Boromir had the vitality to meet the requirement when the choice was made", etc....however, in my experience with the game, whether or not he had the ability to fulfill the requirement when the choice was made, SHOULD NOT affect the fact that the requirement MUST still be fulfilled. Am I wrong?
This is just my take on it, and I could be completely wrong, but to me it seems logical.