What to do about best friends jealous girlfriend?

I 24 (F) have been best friends with 24 (M). Since we were 4 years old. It has always been a platonic friendship. There has never been any flirting or any sexual relationship. We are very close and have gone on trips together, know each other’s whole family, went to college together, and see each other weekly. I haven’t dated since I was 20 and he’s had a few girlfriend. When we are in relationships it does change the dynamic a bit as we like to respect each other’s partners. None of my boyfriends or his girlfriends have ever had an issue with our friendship.

But he’s been with this girl for about 6 months now and she is extremely jealous of me, but also in general of other woman. My friend and I always work out together in the morning and when he started dating this girl she started coming with us and I would say we all got along, she eventually couldn’t go at the same time as us so we kept going together. This is because that’s the only time both of us can go together. She was not bothered but nos suddenly she is bothered by this. He sends me flowers every valentines since I’ve been single. I guess he didn’t tell his girlfriend but she saw a charge for some flower shop and confronted him.

She met his family a while back and I was there for his brother’s birthday. She saw how well I got along with his family and she was mad. I’m not sure why this would make her upset if I’ve known them for 20 years. She said the family would talk more to her than me, but she was sitting apart and I told her to come join us and she didn’t want to so she stayed with her sister. Their family invited me on their family vacation this year as they were going to my home country in Mexico and wanted said it would be nice to have someone to translate. He invited her but she could not go. And this upset her.

When we are both single we usually would hang out weekly and grab dinner or sometimes hang out a few times a week. Since he started dating his girlfriend, or when he dates or I in general I usually suggest us hanging out is a group, which we would do.

So about 3 weeks ago he had an issue which has to do with a secret that I only know about. He has not told his girlfriend so he came over to my house to talk about it. I guess his girlfriend saw his phone and got mad and said he trust me more than him and that he’s probably in love with me. She started to bring up all these things about how we workout alone, how his family likes me more than her, how they took me in vacation, how we have inside jokes, etc. Then she went on to say that he stares at me like I’m the love of his life. She said to cut me off and he said he would not do that and if she won’t accept it they should just break up. They are still together but I haven’t seen her. I’ve just seen him at the gym. I don’t know what to do or how could I make peace with her and let her know are friendship is 100% platonic and nothing will happen.