Sounds like a complicated situation...

Something that your daughter may need to consider is what happens if their friendship sours? ......... and the whole thing ended up in court with a custody battle, and a judgement for 50-50 parenting.
Had not thought of that point - How to get her to realise this is the problem - 18 year olds seem to know everything and do not take advice too easily.

BTW - Hope your treatment is successful