That's scary.
Scubapro for example has a dealer cost on new plastic second stage cases ranging from $3.50 to $5.00 and it does not take too much longer (if any longer, depending on the design) during an annual service to remove all the parts and install them in a new case. The low cost of the case also means dealers can afford to stock a few for each model they are likely to see without breaking the bank. So $10.00-$20.00 should cover it and it is well worth the cost.
So in my opinion, there is absolutely no reason to ever put a cracked case back into service. Glue just will not cut it and creates the potential for the case to begin leaking at inopportune times, often with just enough water being aspirated by the diver to start an involuntary coughing/gagging reflex under water - a situation that can make it extremely difficult to switch second stages and one that can quickly lead to panic.
Some cracks form because of age, becasue the reg got banged around, got swung on the end of the hose and impacted the side of the tank, had a tank fall over on it, etc, but others occur when the tech tries to get the inlet fitting loose from the second stage. If you don't hold the inlet fitting securely to prevent it from turning in the case, the torque needed to loosen the fitting will often cause the case to crack in the square corners of the hole for the inlet fitting. If I slip and do that to a case, I'll absorb the cost of the replacement as it's my fault not the customer's.
It could be your LDS does not have access to Oceanic parts. Many parts (low pressure seats, o-rings, etc) in a standard downstream designed second stage are pretty generic and you can often do a quality annual service on a brand that you are not a dealer for, but that is no longer the case in a situation where you need something like a new case or diaphragm. In that situation, the proper response is to advise the customer that you can't do a proper repair and to refer them elsewhere (or sell them a new one if they prefer). Doing a half baked job is just not acceptable.