Your friend’s relationship may turn out to be the kind of learning and growing experience a 19-year-old is seeking. In any case, if you object strongly to this relationship, end yours with your friend.

I had to do this in the case of two very close friends who, it turned out were cheating on their spouses with each other and allowing me to unwittingly defend their relationship as innocent to others who questioned it. It broke my heart, and I still miss them. I might have been able to get over it if they hadn’t used me in that way. That was the worst part.