I'd be careful about calling this guy a "boyfriend" to her or anyone. Just call him "infidelity partner".

When you call him boyfriend you validate what she's doing. Every time my husband called my infidelity partner an infidelity partner it stung.

I bit my lip each time, but it did sting.

And for what it's worth, this guy is not in any way her "friend."

Make it known how you feel with your word choices. We do recognize the distinctions even if we don't say anything.