"It will bounce from the bank and the client will get a check"

Or the person that actually has that account number might be getting an extra "stimulus" payment this year.  

Once a return has been accepted, bank numbers cannot be changed.

If you are using a Bank Product, most of them have a procedure to make corrections.

Have the client notify their bank, just in case the incorrect account number belongs to someone else. No guarantee or obligation on the banks part to prevent anything, but it's worth the call, I believe.

Correct your file info as instructed and pray there is no active account, at that bank, with that number. The account holder is under no obligation to give it back and the tax agencies & bank will not take any action to recover the funds, to the best of my knowledge.

 I always tell my clients, "It's the most important number on the return, you sure it's correct ?"