Only read the first page so far.
1) YOU did not see him with someone else, the kind of kiss, etc. At this point it is nothing but gossip. Do not talk to anyone else about it; period. And do tell anyone who brings it up that passing that information behind someone's back is at best hurtful.
2) IFF lots of people are talking about it, it would be a kindness to tell your frined that you have heard people talking about her and her boyfriend. If she wants to know what is being said, she will then ask, and you can tell her what is being said, and that you have chosen to not repeat it to anyone, but thought she ought to be aware of the gossip.
3) If you see the boyfriend with another woman, tell him that he must talk to your friend--make a point of letting him know that you will not lie for him or to her. You don't even have to specifiy whether you will tell her or not, just let him know that the possibility is there. That way you have not backed yourself into any corners.
4) Do not attend any social occasions where he is with another woman and still dating your friend.
5) It is possible that he and your friend have broken up, making this entire series of speculations moot.