This doesnt make any sense. If you are married to a us citizen you qualify for a green card. Then you need 3 years for citizenship. You never need to leave the country. Ive been through this process, this is absolutely wrong.

The only way this makes sense is he entered the country without inspection or something is very very wrong with his backgrounds. I personally know many people who overstayed their B1/F1 visas and then do AOS to green card via marriage, none was required to go back to their home countries.

OP either intentionally left out some crucial details or their friend lied.