First of all, it's worth asking whether his Christian morality was focused upon the desire to encounter Christ in the faces around you... or whether it was all about getting assurance of salvation (and acting nice to reassure oneself that one is part of the elect0. If the latter, then his claim is plausible.
But even granting this, I would turn the tables on him in the following manner:
I would change the question being asked. It is not whether good Christians who became non-believers continued to do good or better things, but whether any evil persons came to desire to live genuinely human lives once they came to believe in God AND to contrast that with the answer to the question regarding whether any vicious persons came to do the same once they became thorough-going materialists.
(understatement time): It's hard to see how materialism helps one to discover one's own humanity, one's common destiny with other human beings... it's easy to imagine how the contrary might well be the case.
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