I often hear people debate a very famous engagement survey statement which talks of having a “best friend” at work. 
I’ve never had any trouble with what best friend in a work context might mean … but just in case you were wondering, I thought the following quite apt:

“Your truest friends are the ones who will stand by you in your darkest moments – because they’re willing to brave the shadows with you – and in your greatest moments – because they’re not afraid to let you shine.”

Nicole Yatsonsky