Basically I have a faithless family, and ever since I cut them off my faith is stronger.
They were holding me back, as the pagans also hold this girl back.
I would put this into the category of an evangelical counsel, a supremely praiseworthy ideal, but it's something that also has to be offset with prudential considerations regarding actual situations.
Let's say I had a heretical relative whom I felt that by being around I could possibly bring closer to the faith (someone who posed absolutely no danger to my own faith), or someone who had health or financial problems, someone who had emotional or mental problems. I would still feel obliged to help them even if helping them meant spending significant time with them and maintaining certain social connections with them. If on the other hand, I prudently judged that breaking ties with them would be helpful to make a statement to them regarding the gravity of their actions or if I needed to break ties with them because they were harming my own faith, then that would be a different situation.
So I do consider it irresponsible of MHFM to be giving this kind of advice when they really don't know anything about the real world factors that would weigh into making the prudential judgments that would be required to navigate this situation in the best way. So, for instance, what if the person to whom they gave this advice were prone to scruples or might be inclined to get severely depressed (which could then cause damage to the person's soul). It's one thing for someone to make the best decision for themselves, knowing the pros and cons, but quite another to throw this stuff out there without any pastoral knowledge of the person's situation.
So a prudent spiritual director might realize that in one person's case, they should be required to break ties with relatives, whereas in another's case, they might be much better served (and their relatives better served) to maintain these ties at least at some level. There's no absolute black and white on this matter IMO.