It means when Co-workers become friends, they are "the best" friends. So, yes, it is close in meaning to saying they are the best friends.
Although if interpreted literally the sentence could mean that Co-workers are somehow very skilled at making friends, that is very likely not the intended interpretation.
- Doctors make poor patients
- War veterans make the best guards.
- Lawyers make interesting conversation partners.
- Doors make poor windows.
Sentences constructed like "[Noun A] make [Adjective] [Noun B]" are usually either literally describing one thing creating another (Babies make happy noises) or they are talking about how the first noun does if it tries to fill the role of the second noun. The correct interpretation depends on the context.
Sentences like this where it refers to one thing becoming another might actually be using "make for" but omitting the word "for". I'm not 100% sure that's true, but it sounds right. (I'm a native speaker, and I'm not perfect at knowing how to state all the grammatical rules.)