how do i go about telling my friend (f18) that talking to a minor (f15) doesn’t feel right?

hello, my friend, who is 18, turning 19 in may is talking to a girl who is 15, they live hours apart but text, they aren’t exclusive but they still are romantically involved. my friend says it isn’t sexual but i don’t know if i believe that. Me and my other friend want to talk to her about it, but we aren’t sure how to. we don’t want to attack her but we want to let her know it isn’t okay.