Disclaimer: Trying to avoid a kneejerk reaction here (though perhaps one is really a more appropriate response).
Representative of his character: not great. It's not end of the world stuff, but it is the sort of thing that would really make me question whether it's worth associating with a person (though more from the attitude you seem to be describing than the act itself).
Actual damage done: unless this is a particularly insecure woman, probably pretty damn low. Most of the women I know would be very pissed off for a while and might break off a friendship, but this is probably not life-ending for the average woman. If it was a close friend and a one-time thing with an apology, it's pretty far from unforgivable.
At the very least, you are not overreacting. Turning him over to the police or some such would be overreacting (personally, I'd say this would be massively overreacting for the girl too, but either way that's her prerogative). I would be very angry with him and I would very definitely demand that he tell the girl he did it, a conversation that certainly wouldn't be likely to go well, but might not go quite too terribly if they really are best friends (if they know each other so well, I doubt it's a tremendous surprise that he might be interested in her and this could be less a massive violation and more a particularly inappropriate way of expressing something he's too shy to admit). Hell, I've heard weirder stories of how friends discovered that they liked each other. Of course, if he still thinks this was appropriate behaviour (perhaps in that brotastic high-fiving sort of way), then that's a whole other story.
TL;DR: He fucked up, it's probably not world-ending, and if he owns up to it and apologises you should probably forgive him.
Representative of his character: not great. It's not end of the world stuff, but it is the sort of thing that would really make me question whether it's worth associating with a person (though more from the attitude you seem to be describing than the act itself).
Actual damage done: unless this is a particularly insecure woman, probably pretty damn low. Most of the women I know would be very pissed off for a while and might break off a friendship, but this is probably not life-ending for the average woman. If it was a close friend and a one-time thing with an apology, it's pretty far from unforgivable.
At the very least, you are not overreacting. Turning him over to the police or some such would be overreacting (personally, I'd say this would be massively overreacting for the girl too, but either way that's her prerogative). I would be very angry with him and I would very definitely demand that he tell the girl he did it, a conversation that certainly wouldn't be likely to go well, but might not go quite too terribly if they really are best friends (if they know each other so well, I doubt it's a tremendous surprise that he might be interested in her and this could be less a massive violation and more a particularly inappropriate way of expressing something he's too shy to admit). Hell, I've heard weirder stories of how friends discovered that they liked each other. Of course, if he still thinks this was appropriate behaviour (perhaps in that brotastic high-fiving sort of way), then that's a whole other story.
TL;DR: He fucked up, it's probably not world-ending, and if he owns up to it and apologises you should probably forgive him.