so are you saying to just feed the prepared food for now then until there is an improvement??
The post above this by Topbell (i think) is decent. Once a week is more than enough for items like prawns and bloodworm,the basis of the diet should be a good pellet food, like Reptomin!
As for damaging the water as it wont feed, make sure you remove all uneaten bits of food.
All good advice so far - someone did touch on vitamin A deficiency, its worth considering too. If the person you've had the turtle off hasn't bothered to clean the tank, its unlikely they've offered the correct diet. Vitamin A deficiency can cause the eyes to swell and close, the turtle/terrapin will be listless, unwilling to feed and will often 'beach' itself on any landing bay you have, or hide away underwater. This does need urgent treatment, when I rescued my RES, she had acute vit A deficiency which required a course of injections. 6 weeks later, she's was fine and eating again, but it was horrible to see her so poorly.