Intense relationship during separation

Hi,

I'm actually not here for me but for my best friend. She (41F) realized there was no future for her and STBEH of the last 20 years, and announced her divorce/separation to him in January of 2026.

When she decided to divorce him, we spoke a lot about how she was feeling and what was her plan for the future to "rebuild herself". A lot of it was centered about her needing alone time to find out who she really was post divorce.

Lately, she told me she started having sex with a man who is in an open relationship. One thing of note; she still live in the same house and sleep in the same bed every night as her STBEH until they move out of the house in May. Despite not being her initial intention, she started to develop some feeling for him. The way she speak about her feelings, it's very intense, it's something she "never experienced with anybody in her life".

This sounds to me like a rebound relationship but I want to be supportive because if she is happy, I am happy for her. I'm also very afraid for her which is the reason I'm posting here.

If it was just that, I wouldn't have posted here, but there's more. After she realized she had feeling for him, she talked to him about it, about the possibility of seeing him more often and maybe even start a relationship together at some point in the future. The way she told me how this went, it was not pretty. He pretty much said to her that he was happy being "celibate" and didn't want to leave his current partner to get into an exclusive relationship. Let's just say this was a huge let down for her.

A couple of weeks passed, she is now talking about being open-minded about her next relationship being an open one. It's something she never seemed inclined for in the past, and I am afraid that she is trying everything she can to get that guy to be in a relationship with her, even by doing things she wouldn't have done in the past. I'm afraid she's repeating the same pattern of putting her "partner" priority first, but I don't know what to do about it.

On one hand, I want to be supportive, but on the other hand I feel like this relationship is going to crash and burn and she's gonna get hurt bad.

I don't know what to do other than just being there for her. I don't have any other divorced woman to ask if this a normal part of a divorce/separation. So I'm asking you, should I be concerned? or just let her experience the good and the maybe the very bad from that relationship?