I would cut down the feed - let her graze and have her 1-2 haynets at night, lower calorie hay could be better though, so she can eat as much as she wants, but does not get fat.
Definitely find out what the other person is giving. And probably stop it. You might need to give something to feed her supplements and ration balancers, but there are plenty of low calorie feeds to do that.
So you are the only person working her 3 times a week? Keep doing what you do, when you feel like it, up the work a little. Start by trotting more, cantering a bit more (work over 1 h), then increase the canter work, more poles etc.
Do what you feel comfortable with when it feels like it. You can usually tell when the horse is ready for more work, sweats less after the session or just still feels energetic.