I mostly agree with mjeaton. What you have done is probably unethical, but I think if GRWBSC ends up having a problem with it, it is your client who will get the heat. If they get in trouble, they may end up blaming you, and that's not good even if you don't have legal culpability (which you may or may not; IACNAL).

I would suggest that you point out to your client that GRWSBC may be very irritated if they find out that they have been deceived (by omission if not commission), and that between you, figure out what to do. The right thing might be coming clean with GRWBSC, and it might be porting like you know they wanted in the first place (though that will certainly take a long time and cost a lot; who will pay?). Make it clear to your client that this is a problem and it is currently in their lap.

Scott

UPDATE: given gryphon's comment below (that this is the 4th project done for GRWBSC, and the previous three had been using LAMP solutions, I don't see that what he did was particularly unethical. The rest of what I wrote still stands; having a contingency plan is a good idea.