We were ready to set up a one-man DB Plan (owner and spouse) but he mentioned that he has another employee. She works for him however, is paid by a different company. The client reimburses the other company for a portion of her salary. Seems like she would be defined as a "leased employee". Assuming this is the case, can she or can't she be excluded from the new DB Plan for this client?

In the scenario you describe, the plan wouldn't pass coverage testing if the only eligible NHC is excluded.

Sounds like a shared employee. A "reasonable" approach would be to cover her at whatever portion of her salary is for services to the sponsoring employer.