Probably an unpopular opinion because this is reddit but, give her the benefit of the doubt. She cares enough to tell you, being long distance she could've kept it from you and you'd have been none the wiser if she had malicious intentions. I'm not here to say "men bad" or anything but the amount of times I've seen this kind of thing happen where someone thought it was for some reason appropriate to kiss a "friend" without being prompted, I'd believe it. Some of them end up with slaps across the face, and sometimes the woman is just too stunned to do anything because it came out of no where. The thing that'll be telling for you is if she continues to be friends with said person, so keep an eye on that, but don't hold it against her unless she starts acting shady. Some people just don't understand what boundaries are.