It's about respect, or rather the lack thereof, shown toward the son and SO by the mother.
In order to have had any real choice in the matter, the SO should have been offered a genuine opportunity to decide, which she was not.
The guy is in a tough spot--disrespected, his SO disrespected, yet still caring enough about his mom's feelings that there is even any question in his mind of what to do, and how to do it.