yes!!!!!!!!! shes your best friend..i don't know why your asking this question..you should have already told her!!! don't you want her to be happy and unlied to? as your best friend she doesn't deserve an a**hole of a boyfriend..totally snitch!! its understandable

Absolutely. I would want to know if I was dating someone and a friend of mine knew they were cheating on me. No doubt about it.

Yes, I am afraid you must tell her. It will be very difficult to do so and be prepared for her to feel angry. Spare her feelings as much as possible and don't tell her all the horrible details such as 'multitude of women', cut it down to several at the least. Enough to get the message over and no more. Try to be as tactful as you can and don't whatever you do, pass judgement on the BF. Don't say anything about 'BF being a lump of shit and how could he do it to her'. Just stick to the facts and no more and do not be drawn into making assumptions or into givng her your opinion about him and whether she should be with him. Don't mention anything about how she feels about telling him of her love or any of that stuff. Just say to her that there is something you have to tell her that you think she has the right to know about. Tell her what you know and finish the conversation as soon as possible. Then leave her to make her mind up about what she's been told. She will have a severely dented pride about this and will not only feel hurt but also embarrassed about it. Afterwards, do not bring the subject up unless she does first - period. And, if she decides not to believe you (and she might), then just accept it and say nothing more of it. I don't envy you on this one. Get this over with as soon as you can. Oh and she will ask you how long you have known about this - think about your answer and have it prepared. If you say too long, she will ask why you have let her carry on making a fool of herself and ask why you didn't say something sooner - this question could do much damage to your friendship, so watch out for it. Perhaps you should consider saying others have told you they have seen him out and about and more than one person has come to you and told you which is why you have not spoken up sooner. Refuse the give names of these 'others', point blank. Say 'names do not matter, I am just telling you what I have been told, I thought you had a right to know. Don't ask me who told me these things because that knowledge will serve you no purpose and will not help at all, it will just make matters worse'. And stick to it.