What would you do? Someone needed to have an ITIN application prepared for a joint filing with a non-US citizen...

What would you do?

Someone needed to have an ITIN application prepared for a joint filing with a non-US citizen, non-US resident spouse. The tax return which is filed along with it was prepared by another accountant.

This happens all the time. Often, their US based accountant who isn't proficient in expat laws fails to claim the foreign income exclusion or foreign tax credit, and leaves out Schedule B for foreign accounts. They forget to ask about FBAR reporting as well.

Worse, sometimes they claim the foreign income exclusion instead of the foreign tax credit, often resulting in zero refund due to the loss of the child tax credit.

I could either:

1) Let them know what to tell their accountant in order to fix it, resulting in extra work for me in order to explain it but a happy client who knows I'm not squeezing him for every penny after he already paid his regular accountant. 

An added detriment is that this often ends up with some back-and-forth coaching for the accountant, with no material gain for me.

2) Tell them cryptically that there is a way to get a much bigger refund/pay less taxes, and that I'll explain it only after receiving payment for redoing the return. This would basically force them to pay me, but to me it seems borderline unethical.

I have chosen option 1, and can happily say that many end up using my services on their own accord the next year and forward. 

If I had chosen option 2, they might just begrudgingly use my services the next year. Or, they may send the return I had prepared previously back to their old accountant for reverse engineering, after I had failed to make a good impression on them.

Thinking long term and keeping to a high ethical bar pays off in the long run for both of us.