what do I do if I think my friends boyfriend is cheating on her what do I do
Thinking and doing are two very different things. Make sure he is cheating but not in the way that you con him into cheating by hitting on him or anything. If you see him actually kissing another girl or w/e tell your friend what you saw, but if she has a angry response saying your jelous of her or w/e just tell her that you were looking out for her because you care, and that you will be there for her when she needs you. Just be careful with what you say.
Hope this helps.
I would not say a word till that that cheating is not real a scandalous one. I do not want to be a body guard of anybodies relationship. If somebody would say me that my g/f is cheating on me, I would answer that, it is better if everybody deals with his own love, and not sinffing after somebody's else.
Dont say anything until you are 100% sure!
...then make sure she dumps him!