My sisters dog is on prescription Hills for his joints but only started it. It's really expensive though. Roughly 42 euro for a smallish bag. Hopefully it works.
Yoltan, point your sister towards the following site: www.chemistdirect.co.uk. They sell the Hills prescription food at a far more reasonable price than I've seen it here. My dog is currently on one of the prescription diets which costs €78 for a 12kg bag from my vet's, but only £41.99 + £9.95 delivery from this site. They will deliver up to about 20kg to Ireland for that price.
No affiliation with the site - just a devoted dog owner trying to save a few shillings while still doing the best she can for her dog!