At 19 years old I would say let him make his own decisions. If he wants your advice, give it to him but don't push him to do something he is not comfortable with. These situations devastate kids at any age and it's part of the consequences your W needs to face. Maybe the loss of time with her son will help her realize what she is giving up.
Me 47 W 42 T 24 yrs M 18 yrs W living with OM BD1: 3/7/2015 (A with OM#1) BD2: 4/11/2015 (A with OM#2, W moves out) WW filed for D, papers received 9/18/2015. Meeting to determine child custody 9/29/2015.