Can I suggest to my best friend that they should break up with my cousin?
Why would you suggest they break up?
I think my cousin is nowhere near good enough for my best friend.
It's not your place to interfere in someone's relationship. You should let your friend decide for themselves whether they stay in a relationship. Interfering might hurt your relationship with your friend and cousin.
Okay, I see your point. Plus I think it's going to fail anyway.
I'm glad you're taking my advice to stay out of their relationship, because it should be their choice whether or not to continue it. If it fails, let it fail on its own.