Hi there,
agree with other posters, there is no restriction on who can travel with a bad back or any dissability as long as pax can look after themselves. For any pax who cannot look after themselves, they should travel with a companion who should be able to help them.( help with toilet, eating etc)
As pointed out, to sit still for a long time with a bad back might not be a very good idea. Seek medical advice. Although a business class seat (bed) would be much more comfortable, this is not offered by the airlines. If you want it you pay for it.
As crew we will always try our best to make any pax as comfortable as poss, but we do not upgrade.
If any pax has any probs or concerns the should tell the crew when they board the aircraft. It is much better we know of a situation early on, so that if there should be any probs later on , at least we know we are dealing with.
Hope your friend gets to NYC ok and is able to enjoy the Big Apple!
Trolleytop