The 2017 return should never have been filed as single since your client was married in 2017.  It has long been established that martial status under §7703 is determined based on laws of the state in which the marriage took place.  Immigration status, legal or illegal, and whether the spouse has a US TIN is irrelevant.  The 2017 return will need to be amended regardless; luckily, the statue of limitation for electing to file jointly has not yet run out.

In order to file jointly though, you need to determine what the spouse's US tax status was, based on SPT, before receiving the green card and stepping into the US for the first time (if the spouse was not already in the US waiting for adjustment of status) as §6013(g) or (h) may be necessary.

Since the spouse is now eligible for SSN, she will not qualify for ITIN.  Your client's spouse must visit the SSA to apply for one before filing the 2018 return.