Weight loss is extremely common in older horses.
Of course she isn't going to gain weight on grass- she has no teeth to chew the grass with! The same with hay- she may be able to eat it, but without being able to chew it well, she might not be able to digest it.
Have you tried a complete feed? They are designed to be fed without hay, although I would leave at least some hay to keep the horse busy throughout the day. You probably need to feed 12-20 lbs of feed per day- split into smaller meals, as you risk colic feeding more than 5 lbs per meal.
Here is a good article on complete feeds 7 Tips for Selecting and Feeding a Complete Feed | TheHorse.com
One more with info on feeding hay cubes: http://www.esc.rutgers.edu/publicati...iles/fs073.pdf
You could also try adding oil which is dense in calories. My horses don't like corn oil but will eat canola oil from Walmart.
You can give a cup of oil per feeding, but I would reduce the grain if adding oil. One cup of oil is about 2000 calories.
Before investing in the added feed, it may make more sense to pull bloodwork first. If her bloodwork is okay, I would assume the weight loss is due to the teeth issue.
If she fails to gain weight with a change of feed or continues to lose weight than it may be time to say goodbye.
Does she have a stall or shelter for the winter? I would take that into consideration as well. Is she otherwise comfortable? a horse can be skinny and still be comfortable. Do you plan on blanketing this winter?
If she seems happy other than the weight loss, I would not put her down just yet. Wait a few months, see how her weight does on the new diet, and than decide.
I've dealt with lots of older horses. Many will pass without needing the vet up, but some aren't so lucky. With her heart issue, I would expect her to pass quickly if she does pass naturally.