I don't have a problem with that advice as long as the pump is easy to get to in his car. If the pump isn't easy to get to in the car then I'd swap to the HV pump now. If it is easy to replace in the car then nothing wrong with running the engine and seeing what it does. Adding a shim won't help if the problem is too little volume. If the internal leaks are bigger than the pump can handle then the bypass could be shimmed shut and it still won't build pressure. So if it is a volume thing due to the EDM lifters he'll need the HV pump. Since nobody knows what the root problem is the OP will just need to experiment.