Understood.
Being inherently lazy, if I were faced with the same requirement I would look at developing a Report. I developed a report for users to "see" both their assignments and the "Need Date" of the PBO together. The approach here would be similar... SELECT on Work Items with CURRENT_USER as "Owner", join with Process, joined with Work Items with Completed Status AND [some_user as Parameter] as Owner. It would probably take a bit more to get only where the some_user was specifically the Owner of the last completed... but would get you a return of some_user as Owner has completed a Work Item on the same process.
I wrote out what I thought would need to happen and then cut it from my response as it was fairly convoluted when I read it. If the report does not work for you let me know and I'll share my convoluted concept.