not with a bargepole . . . . . . ! !
Niknak actually summed up most of the important points here very well. I did air ambulance work for many years when I was much younger, and I can tell you that the trip you were contemplating could have turned out to be a much bigger catastrophe than you could ever imagine !
One has every sympathy with the family's wishes (and the patient's, of course !) but it's really a very specialised area of transport, and should be left to those with the knowledge, equipment, suitable aircraft and expertise to do it properly.
To give you just one example of the sorts of cr*p you can encounter : if the patient was really terminally ill, and being returned from Spain to UK, what if he were actually to die while you were in French airspace ? Legally, this is tantamount to dying in France; yet, legally, he has not yet entered France ! ! Bureaucracy rules, OK ? !!!
That's just a taste ! I really wouldn't recommend it, however much you'd like to help out your friends !