I'm not sure that I agree with that.
As long as they seal the cylinder, you should be ok. The cylinder doesn't care who makes the rings, as long as they fit right and seal properly. As long as they are the same type (cast iron vs moly).
Yes, it's more desireable to have a full matching set if possible. I would personally bite the bullet and buy a whole set myself. But if he's on a tight budget, then go with a single cylinder pack.