I'm not going to tell my friend that he might have an std
Why wouldn't you tell your friend that?
I'm trying to protect him from embarrassment
You would better protect him by telling him about the STD. That way he can get checked out at the doctor and receive the help he needs.
Why would I do that? That's his business.
If he doesn't know then it's not really his business yet. You should tell him privately if you're worried about others knowing.