Is he Jewish?

Nowadays everything is possible. Yesterday one of my FB-friends pointed my attention to Mikhail Prokhorov bio-facts.
According to Wikipedia, "His maternal grandmother, Anna Belkina, was a microbiologist and was JEWISH"... I was quite shocked. Prokhorov is Jewish?.. He is. Actually not many Internet sites talk about it. Usually talks about Jewish roots of Russian oligarchs take a lot of space in cyberspace. I'm surprised only couple websites mentioned about his Jewish background. One of the English-based sites defined him as a "quarter-Jewish." I strongly disagree. If his maternal grandmother is Jewish, his mother is Jewish. Therefore he is Jewish. I know, he does not feel Jewish at all, but still it's a great pleasure to introduce new face to our tribe. Welcome a nice bachelor!