With the number of elderly people on the increase, I cannot see why people are so blasé about calling an ambulance if someone falls. The likelihood of an otherwise healthy person breaking their back or neck with a fall is minuscule. "Don't move them" is good advice if someone has fallen headlong down a flight of stairs, or off a ladder and are unconscious or screaming in agony. But for the majority of falls, it is perfectly acceptable to help someone up.
Our daughter has cerebral palsy and falls all the time. If I called an ambulance every time she fell, they'd be permanently camped in our drive. And, yes, after years of having to lift her my back is fucked, but that's what we do for family. If I was that upset about hurting my back, I'd spend more time learning how to lift her properly without hurting my back.
Of course the NHS gives their staff advice not to lift patients alone and to use hoists. This is because they have a duty of care to their employees, and are liable if someone is injured. The advice is generally because of the health of the staff, not the comfort of the patient. Having an elderly relative be forced to lie on a ward toilet floor for an hour when they fell because "the manual handling team" were busy, shows this is never about patient comfort.
To be whinging about a sore back, when MIL is caring for FIL daily and seems not to be getting the right help to do so is incredibly selfish. If I saw someone fall in the street, of course I would offer to help them up, and help them home, and help them any other way I could.