Firstly, even non-frum jews hang up mezuzas!
Secondly, how do you know so much about this guy?
Even goyim advertise in Jewish media. That is just smart marketing.
And as I said, having a minyan in his store is a service to his neighbors. So in what way is he pretending to be frum? He wears a yarmulka and tzizis? How old is he?