Shires and WeatherBeeta rugs I've found pretty reasonable (I got a large, shark wither horse as well and these two brands fit her really nicely). I just order them off ebay.
And seconded ... they need access to fresh air, sunlight, the ability to shed, dry and scratch. I hope I also am not thinking that you seriously would leave a rug on 24/7 for months at a time... o,o
Get your mum a weather and temperature chart for rugging horses or make your own.