Try to Gather Proof/ Show Her
If you have yourself seen the boyfriend cheating on your friend with another girl, you can gather proof about it. Gather any possible screenshots or take a picture while they hang out. You can even take your friend to the place you have seen the boyfriend come with the other girl often. This way, if luck helps, she can see it for herself or know by the proofs.
Send Proofs in Anonymity
There are significant chances that your friend might blame you or cry if you tell them about her cheating boyfriend; and you cannot risk losing her of course. In this case, you can send an y pictures or screenshots you have anonymously from another account or leave any notes in the locker, windshield or room anonymously.
Be Honest and Direct
When all else doesn't work, you can always be direct and honest to your friend and tell her about the bad news that her boyfriend is cheating on her. Try to tell her this news when you both are alone, so she does not feel embarrassed or break out too aggressively. Never add to the bad habits of the guy to make her feel more hatred towards him because at this point, your friend is feeling bad enough.
Let her know how it affects You
We often hear jokes about how the girls' friends don't like the boyfriends of their pals. Sometimes it is true, but it does not mean that you can be excited about telling your friend about his cheating, or even to taunt that you told her so. Empathize with her that it hurts to have her friend being betrayed by her boyfriend. It might make this news a little easier for her.