I’ve got to help sort out and re-paint friends bathroom, a lot of the emulsion has peeled off leaving the bare plaster underneath. Any tips to get it prepped before painting?

I was thinking; scrape off any loose paint then go over with either diluted PVA or just diluted paint before the normal coats. I’m worried about the remaining paint coming away once I paint over it though.

I’d sand what’s on there, clean off & probably sugar soap/thinned PVA over it. Proper bathroom paint is pretty good stuff too so that will help longevity of the finish.

^I would do what dave said as well – scrape, sand, PVA/paint/water mis coat and then proper paint coat(s) as needed.

Is there damp IN the walls that is helping the paint peel?

I would concur with the above except for the sanding. Sugar soap will suffice so long as you scrape off the loose bits.

A diluted mist coat will soak into the bare plaster and provide a key for the next full coat, sounds like the mist coat was skipped first time round. Good point about the damp walls, is there an extractor or adequate ventilation?