How can you prove to your friend that her boyfriend is cheating?
You've been a good friend and told her once and she has not
chosen to believe you (she's in denial and in love) and believe it
or not this is not uncommon. You owe her nothing else so let it be.
She has chosen to ignore you or listen to reason so she's going to
have to learn the hard way. Unfortunately you are going to have to
sit back and let this one play out, but be there and pick her up
when she needs some comfort.