THat was not a rescue centre it was just somewhere to perpetuate a horses misery - these places want reporting and closing down - they are mostly run by amateur do gooders who have no clue how to care for a horse and how much it costs
Yes - hay hay and more hay. Introducing too much 'concentrate' can upset the digestive system & metabolism and cause laminitis at this stage
Get her blood tested for tapeworm, encysted worms & any organ damage - some cattle feeds can cause liver and kidney damage in horses that will cause dramatic weight loss but she might have some other problems in those areas.
Good luck with her.