"since he passed two years prior."

Community Property State, Common Law State, was the title/deed in the name of a trust, was there more than the one child for inheriting? What happened to the father's share all these years? It isn't still the father's, 2 years later. Whoever gave the father's SSN seems to consider Dad still co-owned the house. Is that fact?

Once you determine this, you can pursue figuring out the step up in Basis, whether that is more than just your client, if the 1099-S for the father really belongs to some sibling, etc. Why does any of this matter:

For all you know, Mom was sole owner at the time of her death, and client is not the only heir. But death of Mom in this example means full step up in basis.