That is the sort of question an immigration attorney could answer. I know first hand that a good immigration attorney can be expensive but is there a local community organization that can provide advice? We have an organization here that works with the Hispanic community and a couple of attorneys do regular clinics to help out. (I am sure some people engage them at full price as well.) At one firm I worked at, I directed an employee of a client to a community organization. He got himself legal and was able to keep his employment.
Please bear in mind that filing tax returns is part of the immigration process. So correct filing is essential. Anything other than the truth is a felony and makes the beneficiary subject to deportation. If the sponsor is a naturalized citizen or a permanent resident, any incorrect statements can result in deportation or, more likely, revocation of Green Card or citizenship. Either way, it is the same end result.
So, consider this. If your client files MFJ, you need a nine-digit number to put on the return. If your client files MFS you also need a nine-digit number to put on the return. Filing Single or HOH is not an option and doing so can have serious repercussions for immigration.
I am not a lawyer so none of the above is legal advice. It is also almost twenty years since I immigrated. All I can tell you is that I applied for an ITIN as soon as I came back from my honeymoon (in those days, you could walk into an IRS office without an appointment and get it there and then). I applied for a SSN five weeks later. I never had any issues, although I never actually had to use the ITIN.
More recently a family member came here with her husband. She got an ITIN and filed with it last year. She will be filing with a SSN this year and her immigration attorney saw no issue in doing that. However, the category of visa probably matters quite a bit. In her category, I understand going from ITIN to SSN is pretty normal.
Seriously, encourage him to get some legal advice before he goes and makes a decision he will regret.