I'd like to add that there is a reason we say "fall in love". Also in French an extra-marital female lover is called a "mistress".
You don't know what is the exact nature of their relation, where the power dynamics stands. You don't know what the guy had to do, what kind of agreement they found. It's possible that he lowered himself on the floor in ways you can barely imagine :
View: https://youtu.be/jVJ9tOL28Qw
It's possible that he is being used, and that he is completely OK with that, because he is in love and wants to be used.
"some of them want to use you, some of them want to be used by you..."
View: https://youtu.be/c2_NS6nAPy4
At the end of the day, we can't tell you how to live your life. You have a destiny, nobody can control it, not even yourself.