He is a friend of.....

...best, and, what is more, perfectly idiomatic, to use the double genitive when “one of So-and-so’s” is what you have in mind:

a student of his (that is, one of his students)

a student of Einstein’s (that is, one of Einstein’s students)

Then you have the liberty of writing “a student of Einstein” to mean by contrast either someone who is working on the great theoretical physicist as a scholarly subject or, more broadly, someone who is a close observer of Einstein and his work.