Yes (coming from a girl), You should just tell her you are suspecting something and maby SHE should check it out!
You should tell your friend that he is cheating its best that she knows.
cheat on him with his best friend! then dump his gay self!
You would not like it if your best friend cheated on you with your boyfriend so that is not a nice thing to do to your best friend. First, you should stop saying you are best friends with the girl when you are 'cheating" with her boyfriend. Best friends do not do that. Second, you should stop going out with her boyfriend. Third, you should tell the girl that the guy is cheating on her, even though you will need to tell her that it is with you. If he does it with you, there is a real good chance that he would cheat on her with someone else. Finally, both of you should dump the guy.
Stop cheating with her and forget the whole thing ever happened. You might want to bring it up in 20 years, or so, while you and your best friend are having beers and the girl is a distant memory.
you get divorced and lose a best friend or talk through it
Do nothing. It is between them.
by asking your best friend to spy on him
You obviously listen to your best friend. your best friend wouldn't lie to you. unless your best friend has a crush on your boyfriend. if he is cheating on you, break up with him because once they cheat, they never change and will cheat on you again. there are plenty of other boys out there, no matter how much he loves you!
Have you tried speaking to them? Sometimes the harsh approach like just be honest is the best approach, if your tip toeing around it, it may only cause them to believe your misleading them, thinking your their friend. And the fact its an ex, well your best friend needs to be the one to stand up to them and say the real.
Ask them!!! And if you are wondering they probably are.
Simple. Break up with your boyfriend. Rather that than cheating on him!
Observe and think. Consistency is the best indicator of truthfulness, just as inconsistency is the greatest warning of deceit.