You can send in a letter, yes. As to whether or not they will do anything about it, that depends on what you have. Many marriages just fall apart and some of those, the U.S. Citizen spouses want to get back at the immigrant spouse any way they can including alleging that the marriage was false. So to avoid them thinking that you just want revenge, you would need some good hard evidence. I know this may not be what you want to hear, but as I asked before, please do not shoot the messenger. The more evidence you have, the more chance that they will do something about it. Also, they may wait until he either travels or applies for someone, or applies for U.S. Citizenship.
As far as the Affidavit of Support, divorce does not release you from liability. You are released when one of the following happens:
1) He works 10 years in the U.S. paying U.S. taxes. If he stops at 9.9 years and never works again, you are stuck.
2) He becomes a U.S. Citizen.
3) He loses Residency for some reason:
a) Being outside of the U.S. for too long.
b) He obtained it through fraud.
c) He commits a serious enough crime.
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