I remember some friends implementing something for a business in Turbo Prolog, but I thought that had some non-standard extensions.

It did... it was compiled, and had some of the standard stuff that came with Borland's other Turbo* languages (like screen manipulation routines). Which came with a price... Turbo Prolog code is pretty much incompatible with any other Prolog version due to its use of static typing and Pascal-ish declaration sections for domains, predicates, clauses and goals.

For those who are interested, I think old manuals (and software) can be found on archive.org.

Turbo Prolog lives on as Visual Prolog: https://www.visual-prolog.com/