@Jason *********
this is why posts asking things for a friend can often run off the rails.
the visa whether gotten inside or outside thailand, the type of visa and/or extension, etc, ALL matter to get good answers
Send your friend to the immigration office where he got his "visa/extension" and have him get a re-entry permit.
Now as far as if he will require onward travel when he comes back THAILAND will not require it as he has a valid visa/extension & re-entry permit
BUT
his air carrier might, and some of them have proof of onward travel requirements in place.
So check with the air carrier, if they say he doesn't need it he's fine to fly in on the return trip