I agree it is irrelevant, but:

1) Married couples switch between filing Jointly and Separately in consecutive years, so merely switching Taxpayer and Spouse on a joint return is something the IRS can easily handle;

2) The client is requesting this because the husband typically makes estimated tax payments, and the IRS mishandled the payments last year because he is the Spouse on the return and his payments were made under his SSN, leading to threatening letters from the IRS on garnishing his wages because the IRS couldn't locate the payments (a situation that happened with multiple clients last year).  I completely understand his perspective here, and ultimately this is the call of the client.