Marriage fraud is a serious immigration offense--However, many facts are missing to determine which waiver, or waivers, if any, are available to waive this infraction. Contact an immigration attorney ASAP.
This is a very difficult situation. First, he'd have to undue the bell that was rung when he plead guilty to the federal crime. Extremely difficult. Then, he'd need to reopen his removal case from abroad. As difficult. Then there would have to be some lawful basis for him to stay. The really bad news is that if CIS made a 204(c) finding (which is very likely), he's permanently ineligible to benefit from any type of visa petition. Extremely limited chance of success.
There is no simple answer and it will take knowledgeable and creative layering to solve your problems. Hire an experienced immigration attorney. Make sure they know what they are doing, it is a complicated matter.