... so like. I just want to be clear: when a romantic partner insists that their partner should not have friends (whether along gender lines or not), that is either explicitly an act of abuse (isolating them from other social structures) or at minimum a red flag of emotional immaturity (ie, jealousy) that will often become a component of abuse. it is not a normal thing. it is, at times, normalized by a culture that expects unhealthy relationships, but it is a blaring red siren for problems.
there is a huge difference between the friend themself choosing to leave you and their partner demanding control over their social circle. it doesn't make being thrown to the curb hurt less, but I do highly recommend leaving the door open for him, and in some way explicitly saying he can always still talk to you, even if you are upset with him. generally, at this point, it's not worthwhile to say "I think that this person is abusing you", because the control is being initiated and is actively monitored at this stage; you saying anything bad is going to be used as why he should cut you off, because "I can't believe you'd be friends with someone who hates your romantic partner!" is a thing.
If you can, please please read up about how to help friends in emotionally abusive relationships. the main point is: be open to listening, never initiate a negative comment about the abusive individual, but if he starts it? Listen. Say how you can see that, how does he feel about it? Does he want to do anything about it? If not, you gotta learn to say "Okay. Let me know if that ever changes." and move on. If he's not ready, it's way more important that he's still able to come to you and doesn't feel pressured, because then when he is ready, he knows who has never pressured him, always listened and let him make his own choices, and offered to help.
