If the government can show that you were knowingly receiving assistance and not reporting your household income properly there is a chance that yo will be charged with welfare fraud. The level of the offense can vary depending upon the amount of assistance you received illicitly.
The penalty for the offense of welfare fraud ranges from probation to jail depending on severity and past criminal history. It is likely that you will be ordered to pay restitution to the state (pay back the amount of fraudulent funds you took).
This answer only speaks generally to the crime of welfare fraud itself and not directly to your case. Hope it helps you understand. If you are being charged you should immediately contact a local criminal defense attorney with experience defending these cases.
Best of Luck!
Welfare fraud is a serious charge. Your fiancé is looking at fines/imprisonment depending upon the circumstances. He needs to retain a lawyer ASAP.