Is it wrong for me to want to tell a colleague's husband she's cheating on him?
Oh are you really thinking of doing that? How will you prove to your colleague's husband that his wife is cheating?
I have a photo of her kissing me
You really shouldn't mingle with someone who is married because nothing good ever comes out of it. Why get involved with this mess? There are too many ramifications that you will regret down the road.
I just want to be a good friend and let him know what is going on.
You have not been a good friend because you were the one his wife cheated with, can't you see that? Your mind works in a way that will get you in trouble one day for sure. Please stop seeing this person and move on without telling anyone anything before it's to late.