There is a continuing issue of how you determine the size of the "household" when selecting the applicable official median income for your state which is important in the Means Test. The bankruptcy statute, 707(b)(6), speaks of debtors whose income is less than the median state income for a “household” of that size. On one side is the "heads on beds" approach, counting everyone who lives there. On the other side is the IRS number of exemptions approach, which says that only those people who qualify as an IRS tax exemption should be counted. The determination of this issue varies by court jurisdiction. If this issue matters in your case, then your bankruptcy attorney must be current about how it's been decided in your jurisdiction.
No reason to add the person. You cannot file jointly and it sounds like you both can file chapter 7. In some districts you can add a significant other for the purpose of qualifying for a 7 but you do not have to add the person unless you are a heterosexual married couple.
Disclaimer: This answer does not constitute legal advice. I am admitted in the States of New York, New Jersey and Massachusetts only and make no attempt to opine on matters of law that are not relevant to those three States. This advice is based on general principles of law that may or may not relate to your specific situation. Facts and laws change and these possible changes will affect the advice provided here. Consult an attorney in your locale before you act on any of this advice. You should not rely on this advice alone and nothing in these communications creates an attorney client relationship.
Since you do not need to have a household of more than one in order to qualify to file a Chapter 7, the attorney is tellilng you that you should only take the one exemption you need and avoid having an argument with the bankruptcy bureaucracy down the road.
While some people think it is fun to get into a fight & win, why spend the money to take a risk that is entirely unnecessary?
Sounds like the attorney you picked is a WINNER!
Hope this perspective helps!