your best friend's partner is cheating

Just make sure that he is really cheating because there is a possibility that if you will tell it to your friend automatically he will talk to her partner about it and there is a possibility that he may deny it. There is also possibility that if the guy can make a perfect alibi even he really do cheat,it may cause that you friend think that you are trying to destroy there relationship. You must also look for some evidence.

I would probably tell my friend.

I think to get involved can be a risk as your friend would not be happy and if their partner gets to talk his way out of it which they sometimes do then it will be you on the outer but then looking at if from the other side I would be angry with a friend for not telling me so I guess it is a very hard situation to be in, maybe as a previous answer talk to the chater first but then that could get you into trouble as well...thats a very hard one...

swissheart I think I would talk to the cheater first if I knew them fairly well.....but yes I would probably tell the one being cheated on..

I would deffinately tell. Friends are supossed to look out for one anther. What kind of a friend would you be to let someone hurt your friend in that.

Oh yes i would. How can your turn a blind eye on your best friend and still claim to care for him. I would also expect him to do the same for me if i was being played.