An issue people seldom consider is that as a disabled guest, you most frequently are required to ride at the back of the bigger rides.
It is common sense for evac reasons, and I am sure you are aware - but if not, consider the extra forces the body will be under. Nemesis and Rita are the main culprits for this.
I frequently escorted a disabled person on rides, though not for the last couple of years, for which the back has now become far too intense. I only mention this as it is something that doesn't crop up very often, but if your friend is prone to pain/aches or stress on the body, it is something worth considering.
Other than that though, you will find Merlin staff and parks on the whole very welcoming and understanding. (Whether that has changed or not in the last couple of years I don't know.)
I hope you have a fantastic day out, Merlin were almost always exemplary in this field.
(Yes, it is me, no one has hijacked my account, and I really did just praise Merlin highly lol).
:twirly: