The idea that today’s non-frum are tinok shenishba and not mumar lhachis is very far from being universally accepted. It is widely held that they’re not tinok shenishbas if they are exposed to Orthodox/Torah Judaism with even as little as knowing about Orthodox Jews. Nothing from the Chofetz Chaim supports the notion that they’re tinok shenishba. And certainly someone who was frum and R”L became non-frum is considered to be far worse.
HaGaon HaRav Binyomin Zilber zt’l explicitly paskens in Sh”utz Oz Nidviru 14:60,69 that it is permissible to speak l”h about non-frum.