The worry here is that what seems like a simple ding on the outside could have caused some delamination of the frame on the inside, which in turn would cause weakness.
In general, a structurally intact laminated frame tube will make a clicking noise if you tap it with a coin. It should sound the same along the full length of the tube. A damaged one will usually make a duller thud. Notice I said in "in general" and "usually". This method is not an exact science and is no substitue for proper analysis if you are really concerned about it.
This next advice may not be useful for your seatstay scenario, but if it is a larger tube you can get access at you could try to take a look inside with an Inspection Camera.
Other than that, if you really want to be sure then it needs better analysis by a professional. Possibly an x-ray.
If it is badly damaged enough that it should not be ridden, you might not even need a new frame. Carbon frames can be repaired.
That being said, for what sounds like just a minor chip I'd go with your mechanic friend's advice and just keep an eye on it. Like Frisbee said, the bike shop is probably worried about liabililty. If it was the fork/downtube/toptube rather than a seatstay I'd be more concerned. If they fail it's not just your frame that will end up cracked. You'll likely have some cracked bones and teeth too.