And why did he not receive a 1099-S?  Did he lie at closing and sign a form saying it was all his primary residence?  (Sometimes people sign these without reading them.) Or maybe they sent one but they didn't have his new address and the forwarding order expired.  If there ever was one. 

Did he live there 2 out of the last 5 years?  An interesting question would be if it made a difference if he moved from one unit to another.  Glad you didn't ask that.  

The rental portion goes on the 4797, and the personal portion goes on Schedule D, perhaps claiming the Section 121 exclusion.  As @TaxGuyBill points out, the allocation between rental and personal depends on facts and circumstances.