“Assuming your belief is that there is no hope for the child to return to the derech, utter rejection is almost certain to create an enemy of the frum community, while setting boundaries in a non-antagonizing way lets you maintain a normal relationship.”
If you know there’s no hope for the child to return to the derech of Torah, why would you maintain a normal relationship with him/her given that he/she is a willfull Torah violater every day without remorse, and having been brought up in a Torah home is no tinok shenishba?