Do I cut off my guy friend who got into a relationship?

My male best friend (5 years of friendship, 2 as close friends) and I are strictly platonic and part of a larger friend group.

For the past year, he’s been dating a mutual acquaintance who doesn't get along with the rest of our group despite several attempts to include her. Over the last few months, my friend has started dodging group hangouts if certain girls are attending. He admitted to me that his girlfriend is uncomfortable with him associating with them. He also said that at first she seemed fine, but over time asked him to cut them off for her sake.

Lately, things have escalated toward me. She watches me closely whenever we are in a group. She invited me to coffee, but it felt like an interrogation where she watched my reactions whenever she brought him up. She hasn't said anything directly, but it's making me extremely uncomfortable.

Now, I'm overthinking every innocent interaction. When he texts me privately, my first instinct is to shut the conversation down. I’m starting to think that she’s going to ask him to cut me off too. However, given how close we are, I’m not sure that he’ll be willing to do so and I really don’t want any drama or problems in my life right now with her blaming me for anything or being upset with me for being friends with him.

What should I do? Do I distance myself, stay close, talk to him, talk to her, or something else?