Love all the pictures, so cute!
Anyway, I had a guinea pig growing up and they are very low maintenance and quiet pets. They naturally groom themselves and are very tidy. As long as the owner keeps their cage clean, there shouldn't be any smell. As someone else mentioned, just be clear about where the owner can clean the cage or dispose of the contents (Hay, sawdust, waste, etc.) Also be clear upfront if you want to impose rules about the guinea pig being out of the cage or on the furniture/floors.
Hope everything goes well. 😃