Im not sure what the situation was prior to her leaving to join army, as the boyfriend has to deal with separation anxiety and jealousy I could understand the wish to spend night with same. It is surprising that the other mother got in on the scheme, I would tell my son to handle his own affairs as my getting involved would be against my principles. However a quick talk with her that any resulting grandkids and mother will be living with her and not you might wake the sorry bitch up.
thanx,
bill