Much in jewelry has stones in it. I've worn opal on me for years at a time (the same opal), and for the last four years I haven't removed my class ring which has a yellow sapphire setting in it. I've yet to have a problem with it.
Stones might have negative effects depending on the stone, what is being done with/to it, and what has been done with/to it. If you are worried about it, I'd convince him to cleanse it. It's pretty easily done.
If you are worried about him being too focused on one stone and not on all of nature, try to figure out why he's just focusing on that one. If he just lacks the experience/confidence with all of nature, get out there and get involved with him. If he's actually just wanting to focus on one piece, then let him.
Voice your concerns to him if you're really worried though. :)