Hi

Question
Can I feed my guinea pig rice hay?

Answer
Yes, you can feed rice hay to your guinea pig. However, it has little or no nutritional value so do not depend on it for a food source. Think of it as just a supplemental additive to the diet that will help keep the teeth worn and give the pig something to do.

These are grazing animals and as such need to have something to chew on all the time. So for that the rice hay should be fine. Just don't use it as the only food he gets.