How to put weight on a skinny horse?

Sweet feed is garbage food for horses so if you can get her on a senior feed, that would be the first good step you could take. Hopefully, she is getting free choice hay and a salt block and clean water as those are a necessity for good health. Since she is low weight, the alfalfa will be good for her and will not cause her any problems. You did not say how long she was working each day. Do you think three days weekly is too much for her right now? Just wondering since she is under weight. If she works a heavier workload with warmer weather, the alfalfa hay will not be good for her and she should be switched to a good grass mix hay. The digestion of alfalfa in the horse's body creates a lot of internal heat. In hot weather with a heavy work load, that would be way too much heat for a horse to handle and would be dangerous (such as working cow horses in Texas cannot eat alfalfa due to their long hard work schedules daily and the heat in their environment). If you could get her on a senior feed and take control of the feeding schedule, that would be a good thing for the horse. It is good for the horse that you are involved in her life and hopefully, can influence the owners and educate them so the horses will be properly cared for. I think the key is getting her on a good food source and give her two to three feedings per day. Bless you for caring about this horse and trying to help her and the owners. Good for you and even better for her.