What can we do regarding a product idea that was stolen?

Since he refuses to cooperate, you're going to need to take him to court. You will need to prove that he stole the majority of his product from you, that he did it deliberately, and that you suffered damages as a result. You can also challenge any patents or copyrights that he holds on the product.

This is an example of why you should hire a lawyer to protect your interests from the very beginning. Your husband and his partner should have had a signed nondisclosure agreement with the man with whom they were sharing their product plans. It is possible that enough of a contract exists within the emails. You now need to hire a good business lawyer to assess if you have a case. This is why I tell people that refusing to pay a lawyer a low amount to write up contracts, read any contracts, and help you set up your business almost always backfires. Trying to mop up the mess you get into by not having your legal work done right, or not done at all, costs far more. The terms of working together should have been clearly spelled out in a contract before anything even began. That would include nondisclosure, confidentiality, duty of loyalty, etc.