occupational acquaintance

Somebody you know thru work (colleague, customer, etc.) with whom you're friendly on the job, but you aren't interested in becoming their buddy/ mate nor in fraternizing with them elsewhere.

Brian: I eat lunch in the employee cafeteria with David three times a week, but when I invite him to a barbecue or to go out drinking on Saturdays, he always claims to be busy or have other plans.
Scott: Sounds like he considers you an occupational acquaintance!