My daughter got a divorce from a husband that is controlling also. However I do maintain a relationship with my ex son in law. I have a grandson and I think it is in his best interest that I do so, so we can keep the communication open.
Your Mum did ask you if you minded, so if you do mind, you should say so. She probably thinks you don't mind, and I'm sure if she knew, she wouldn't be so friendly towards him. So as it is really bothering you, tell her.