Someone I know once tagged along with a (priest?) who had an exorcism/deliverance ministry, when it involved a mutual friend. She reached out to lay hands on the person and try to pray over them— but the experienced one (priest?) made it very, very clear that they (a) didn’t know what they were dealing with; and (b) it was very imprudent to touch someone who has a situation that they didn’t know about; and (c) that you don’t go casually opening yourself up to interaction with something you don’t know about.
So, I’d say fast and sacrifice for your friend. Keep her in your prayers. Offer communions for her, and pray for God to give her the graces he wants to give her the most. But don’t go actually touching her, or laying hands on her, or anything like that.
There was a good FSSP priest I remember, who related one exorcism he’d (been involved with somehow?). There was a girl, and she just wouldn’t be cured of the problems she was having, being possessed by an entity. And so they had talked to the thing that was oppressing her-- and basically, it came down to, until she forgave/came to grips with the situation that had invited the thing into her life, that the thing would never go away.
I thought that was interesting, because it coincided an experience I had with someone who was a genuine case of MPD, back when it was called MPD. She wanted her friends to understand what was going on in her life-- and I was tagging along on the outskirts of that group, so I went along. They triggered one of the personalities to come out and center by talking about it. And one of the things that came out of that conversation was that each of her personalities had been triggered to protect her from some big trauma in her life. When she came to grips with the thing that had created them, they would go away, but until then, they would remain part of her.
So, whether it’s an illness or a case of possession or something in between, a lot of it lies with the individual who’s suffering, and their ability to work their way through the big thing that is at the root of their trouble.