As Taxprohere says, there are various rules and time limits about F1/F2 being exempt individuals whose US days are exempted for SPT purposes.  Assuming they are still considered exempt individuals and NRA's, they won't be able to elect MFJ filing.

Normally, foreign nationals would try to avoid having their worldwide income fall within the US tax net by remaining an NRA.  Is there any special reason why they want to file jointly as resident aliens?

If the wife needs to file a US return, she could be eligible for an ITIN.