Druids are the last living protectors of a very old approach to congregating with nature. Such ways are now only formally passed down in the ranks of the elves, who have based their way of life as a community around protecting nature in these dark times. The knowledge is closely guarded, and so even among elves, they are rare. For an elf druid to take on a non-elf as a student in their ways would be considered treason, and as such no cases of such a relationship are known.
However, it is also possible for someone of remarkable talent to start from scratch and become a druid on their own. After all, the world’s first druids had to learn from nature’s mysterious ways, because there were none to guide them. To emulate these people of legend may be the only path left for a non-elf druid.