For context my partner is polyamorous and I’m open to him exploring another relationship (we’ve been together for a few years and live together) He fell for a mutual friend of ours (he’s always been closer to her than I have) and confessed how he feels to her.

My partner set up her renting my old room from my dad when she was kicked out. The first night me and my partner planned to stay in the living room so We could make sure she was okay and to have a few drinks with my sister. We all had a couple drinks and he kept flirting with her, the first time he did she told us she was speaking with someone and nothing would happen, he kept drinking and flirting and a few drinks later they kissed and he went with her into the bed for the night (nothing happened they just cuddled) while I was on the sofa.

Couple weeks later him and his family took me to Disney world for the first time for two weeks and he spent the majority of the time talking to her or about her, and sent her a lot of things to help her get herself sorted, got her a necklace that cost hundreds and a few other gifts. we both gave her hundreds (he paid me back for the times he asked me to send it to her) and planned a trip to Disneyland Paris with her and I paid off her rent debt, and he paid another debt off (around 500) on the condition she never ask us for money again. She said she was starting to develop feelings while we were away and we got back and she liked the gifts and wore the necklace every day, and a week after we met one of her friends who we ended up also friends with (heather for ease) He bought the girl he likes a bunch of fake roses with notes attached(he spent the week learning calligraphy) which she then just kept discarding and leaving about the house without reading, I had to remind her she left them.

Things were better than friends and she said she had feelings but not enough to pursue anything and was just talking with him. she got back in contact with her ex (they hadn’t spoken in ages and broke up because they decided they were better as friends, this is also where some issues started) and she said she was planning on moving in with him last month and began spending all of her time with him and cancelling plans to see him. Heather started saying a lot of things about our friend using him for money and that she was trying to get with Her ex and things like that (we later found out part of that was out of jealousy) which ofc she denied and told him that wasn’t true and was mad at heather for saying all of that. My partner has come to me a lot for advice and comfort to which I’ve been doing my best to reassure him and he’d vent about wishing someone would tell her something or asking me to find out about but because of other people for example heather and some of his other friends filling his head with stuff it was took on both sides as me interfering so when they pulled me up on it I left them to handle their own stuff

She and her ex moved in together and they began getting close. She told my partner there was nothing intimate etc going on between them which turned out to not be true I got sent a picture of them both shirtless her laying on his chest a week later from her and several other pictures, (nothing of them actually doing anything) which is fine neither of us care about who she’s intimate with while she’s single it’s just the fact that she lied to him about it while speaking with both of them. For context when I knew her ex (about 6 years ago) he kept trying to get sexual favours and nudes from women even after being told no and there was a few rumours over the last couple years about him spiking someone and being involved in a drug batch gone wrong that killed someone. I brought those things up with her before they moved in together purely incase it was true, to which she heavily defended him said the drugs and spiking he never had a hand in and the girls that he’s not like that anymore. It’s become pretty clear that things with her ex are building but I know my partner isn’t going to be able to have a functional friendship as close to her as she is watching her with another guy and he asked me if there should be a bit of distance between them (not non contact just a bit of space). What should I say?