My guinea pig loves to eat string. Is this okay?

So my guinea pig, Georgie, loves to eat bits of my keychain lanyard. Its a lot like hemp but much thinner and lighter in color. Plus the stuff is like catnip for the little guy. He'll purr and sing away when he munches on it. So far he hasn't shown any sickness or anything from it, but i just want to make sure he won't get sick in the long run. He's already 4 and it would kill me to lose him.