Unless someone is absolutely sure that not-Frum people are not to be considered as tinok shenishba, I think that it is assur to let oneself think of them that way. We have an obligation to love them and think favorably of them as we do any Jew.
Also, I don’t think the terms “frum” and “not-frum” the way we use them today necessarily have any halachic validity. If someone considers herself Frum but ignores the prohibition of wearing short skirts or speaking Loshon Hora, even though she knows they are assur, I would think that she would be considered “not-Frum” technically.
On the other hand, if someone did not grow up Frum, but is trying to grow and do what he/she can, I would think that he would be considered more “Frum” than the person in the first example, and I don’t see why it would be permissible to speak L”H about such a person.