There goes the polymath. A major change happens.
The Her <--> him tier is now shaky
The her <--> you tier is now shaky
the her <--> (you + him) is now shaky
and the Her + you + him is now shaky.
You really cannot fix her. He cannot either. We do not control how we feel, much less how another person feels. All we can control is how we BEHAVE in response to feelings. We can REACT or ACT WITH INTENT.
She has to decide to adapt/adjust to the new situation herself IF she chooses too. And how it came about might be giving her some obstacles toward that.
Why? Because she's gotten insecure. Why did she get insecure?
Did you guys include her in this decision? Is the move temporary til you get your own feet back under you and move back out? How long will you be living together? Does she have alone days with him at her place? Has she been asked what would make her feel more in balance or safe emotionally? What does she fear this change will cause? That you two will get kooshier and she will be left behind?
In my universe? People have rights and responsibilities
You cannot be mind readers. You'd have the right to feedback and clear communication. Ask her "Have we hurt you somehow in making this decision? Can we talk?"
She would have the responsibility to know and state her wants, needs, and limits. She would have the right to support and nurture. But she has to spit it out -- whatever it is. You can't even begin to know how
to offer her support and nurture if she doesn't Spit it Out.
She's got to own that piece of the elephant.
You all can sort and everyone try to own other bits in service to the greater polyship that you share.
But that ain't happening if people are not Spitting it Out. Speak your Truth. Even if at a whisper.