Many people in this world face this exact dilemma: a child they love and adore can only be dropped off by someone they can't stand the sight of and don't want to speak to at all. It's more common when the "someone" is the other parent, but the concepts can apply in your case just as well. It may be more work than what you used to do.
First, try to see Alex without getting Mary involved. This may mean going and picking them up, perhaps at a time when someone other than Mary is in the home. Or meeting them at school (with permission of course.)
Second, when Mary drops Alex to you, meet them both at the door. Greet Alex warmly and happily. Alex has nothing to do with this disagreement with Mary. If you can, try to say "thankyou for dropping Alex off" or some other normal friendly thing that you would say to anyone, and half a sentence about when Mary will be back or when you'll be returning Alex or whatever.
Do not say "come in, have a cup of tea!" or whatever you used to say. Go ahead and be cold, but polite. Not pure icy silence, but nothing extra and no invitation. If Mary starts to say anything, such as "aren't you going to invite me in", turn to Alex and say "why don't you take your stuff to your room" or "why don't you get the controllers out and get X launched for us to play" or whatever else is age appropriate for Alex to go and do. Once Alex is out of earshot, calmly say "no, I don't that would be ok right now." Then again thank Mary for coming by with Alex and confirm when and how the visit will end. You are super happy to see Alex and you are a super co-operative adult in the visiting process. You just don't want to talk to Mary at all about anything including why you don't want to talk to Mary.
If she insists on discussing it, you can say that you think another time, when Alex is not around, would be better. Or that you need time to get over the upset of The Incident before discussing how you can move forward. Or that your therapist recommends you don't discuss this with her. Or whatever.
And then close your door.
The variant when picking Alex up is to greet Alex warmly, thank Mary for having Alex ready, and confirming the return time and protocol, then leaving. If you have to wait while Alex gets ready, you can just wait quietly without having any conversation. You can say the same sorts of things about not talking about it here and now.
All the same things apply when it's time for the visit to be over. Have Alex ready in advance of pickup so there's no "standing around waiting" for last minute packing, peeing, etc during which you and Mary aren't talking. Be polite but not warm, and don't talk about what you don't want to talk about.
