What to do when Mom is throwing up her food?

You should check to see if she's actully vomiting food that has gone into her stomach or is regurgitating food that has gotten stuck in her esophagus and not entered her stomach. Older people will sometimes have problems with esophageal movement, so the food does not progress down as it should, or with strictures in the esophagus or the muscle where the esophagus meets the stomach that opens to allow food in. It might require a scoping by a gastroenterologistto see what the problem is, but they can do a balloon expansion of the esophagus at the same time if they see stricture. My grandmother had this done in her 90s and my husband has had it done as well.

If she's vomiting food that's gone into her stomach, could it be medication-related? Changed meds, or changed timing of existing meds?

Hope you can get this resolved soon so your poor mom can gain back some weight.

Has this been reported to your mother's doctor?

Have you discussed this with her doctor?
There could be a medical reason she is vomiting.
How long after she eats does she vomit? And is she alone when this occurs?
A big problem would be Aspiration Pneumonia that can be fatal. As is possibly choking on vomit.
Is she on a soft diet or regular meals? Soft diet might be best until this is figured out.