Is Marty a friend or an employee?

Friend.

They were indeed friends, but it sure is an unusual friendship.

"Odd couple" type friends.

My understanding was that they were just friends.

I think in the original 1985, we see Marty's family as being somewhat dysfunctional. Lorraine is a drunk, and his dad is such a wimp he lets his work colleagues drink drive in his car, whilst he does all his work for him. Maybe the implication is that Marty needed a better adult role model.

Or maybe an eccentric local scientist would just seem cool to a teenager. We see Strickland warn Marty that Doc is dangerous.

I always viewed him as a mentor kind of a teacher type but also a friend. As someone else said his home life in the 80s was considered dysfunctional. So Marty would likely reach out to a father figure type. That’s how I’ve always seen it

They were friends. The word "friend" is mentioned by one of them in part III from memory.

But the chemistry and sheer joy of them performing together made me never question how or why. It just worked so good.

So maybe Marty was riding his skateboard passed Docs garage one day and heard him try out the huge amplifier. Being a guitar player and a curious teenager, he decided to go check it out. Doc being Doc did like he did in the 50's, pulled Marty inside and made him help with some thing or the other and they just happened to hit it off.

I was under the impression he worked for as sort of errand boy. Whatever needed doing was Marty's job.

You also have to take into account that "the original" Doc probably already knew Marty's history and may have arranged with his parents to hire him, without giving away the secret, so as to keep the timeline intact.