Cat losing alot of fur! Any ideas?

Theres a cat I know that used to be a pretty fluffy cat, and now she has big bald spots all over her body, tail really thin looking. She also has scabs all over her body and bumps from scratching it seems. They say that they find little pieces of scab with fur on them from the cat scratching. Any ideas of what could be wrong with this cat? They suspect mange and want to put the cat to sleep. There was a problem with fleas at one point, but its cleared up now and their other 2 cats and dog are completly fine it seems. What could this be, and what would the treatments be, if any?
Not my cat and the owners don’t have the money for a vet

It very well could be allergies….for goodness sakes they need a vet that knows what they are doing. Dont put the cat to sleep cuz its losing fur. Stupid ppl.

The treatments are a variety of things, including oral meds.