I have a cat that started going through the same thing at the beginning of the year. Losing weight, more active than normal, loosing hair, drinking a lot of water and vomiting, not just hairballs. Your mom's kitty needs to get to the vet. to confirm or rule out. If it is a thyroid problem it can be easily managed (when I say can be, I mean for most cats) My Duncan has not been able to tolerate the methimizole, it made him very lethargic and increased the vomiting. We are looking at possibly having the thyroid obliterated with radio active iodine. May be our only option, but again, most cats do very well with an inexpensive medication. Just make sure they do a T4 test. The suggestion on full blood work is definitely in order and, as I have found is a good thing to do annually anyway. Even if his blood work is normal now it will give you a good baseline for later and that alone can save your mom a lot of frustration and money down the road.
And of course, as several people mentioned it can be other things as well.
You mentioned your mothers fixed income, and one thing I would like to suggest is before you go, make a list of anything you (or your mother) have noticed that is out of the ordinary. It can be the key to saving not only money but time and stress on you, your mom and the kitty. You mentioned drinking well. More than usual? More urine in the litter box? Eating well, does he seem hungry all the time? Vocal? More active than usual? Little burst of energy at odd times? Vomiting? If so Does he ever vomit while he is eating? Is the food he vomits digested or more regurgitated or not digested? A lot of people who have cats that vomit hairballs and think that anytime they vomit that is what it is and neglect to mention it to the vet. Does he seem to get cold? My cat always wants to be on me, warmth seeking, and his hair is usually fluffed, nd he always wants under the blankets.
The more info you are able to give the vet the less chance you will be spending money on test you may not need.
I have just been through a barrage of test with both of my cats and there were things I noticed in retrospect that I did not think to mention and even the best vets fail to ask questions that might make you recall something you failed to mention at the start, that could make a world of difference. Sometimes our vets office is so chaotic that I don't remember everything so now I take a list to make sure I cover anything I need to tell them as well as anything I want to ask.
Hope it helps,