Using rocket motors for launching gliders as been around for a long time. I've been into rockets since I was a kid, and have done everything from little Estes rockets (A-E motors) to High Power rockets (H and up motors), and I've seen many people launch and fly different kinds of flying vehicles with rocket motors. Estes makes specific motors that are plugged at the end that have no ejection charge fo that very purpose. ALL Mid and high power motors can be assembled plugged or with an ejection charge. As long as the installation is done properly, there should be no safety issues. I just hope he uses CARDBOARD tubes for engine mounts and NOT PVC. That way, if (or when) the engine CATO's, the shrapnel will be paper and not plastic.

Here's a couple of pics of my largest rocket I've done.