Mom is in the hospital and refuses to eat. Any suggestions?

My MIL drank milkshakes. Not sure about the ensure and boost products but I have frozen slimfast till it thickens like a milkshake. The Royal choc was good this way. Put it in another container and she won't know the difference. It has been said that antibiotics effect taste.

My mother was always a really bad eater and it got worse as she aged. Nothing we said or did (or begged) would make her eat when she didn't want to.  Older people are not as active so they don't need as many calories and the very old have a reduced sense of taste (so food is not as appealing, especially meat)   Lying in a hospital bed and taking antibiotics and other drugs can also have an effect on appetite.  Its not a pleasant environment to begin with.
Try offering  your mother anything you know she likes because even if it isn't nutritious at least its getting calories into her.  Also, one thing at a time might be more appealing than a tray full of food.   My mother was very put off (and actually very upset) when a lot of food was put in front of her.  She was more likely to eat something if she was not overwhelmed with volume and choices.

Mom lived to be 101 and the last few years in AL she lived on yogurt, cookies, Ensure and sweets. There was nothing we could do to get her to eat even half of her meals.  But the  ability to taste sweet things remains and is different than the sense of smell and taste. The nursing home offered her good food, which she wouldn't eat so they gave her fortified puddings. ice cream and sugary drinks.

Hospitals and elderly folks don't get along well. She might have a version "hospital psychosis". Talk to her doctor about ways to ease her agitation. When my mom was in this condition, we asked for a psychiatric consult, which helped us enormously.