Since the 15th century, the word 'marrow' has been used in certain regions such as Scotland and north-east England to mean companion, partner or 'mate'. Its origin is probably Scandinavian and related to a Norse word for 'friendly' or 'communicative'.
The word marrow in the bone marrow sense comes from a common root that applies to most of the world's northern languages, including even Sanskrit and Russian. The Norse version for bone marrow is 'merg', whereas the Norse word for mate/companion is 'marg'. Pretty close, so they may possibly be related.