They are very controlling and try to isolate converts from their friends and families. Sometimes loved ones can be rescued with by an intervention that points out the counterfeit aspects. In this case, I will offer a few points:

1. Your FIL's decision to go to a doctor is ultimately his and his alone.
2. If the church has discouraged him from doing so or has in any way attempted to restrict his freedom to do so, they are practicing medicine without a license and are legally liable.
3. The Book of Acts was written by a doctor. That is to say, the most comprehensive record of the work of the Holy Spirit in all of human history was written by a doctor who had not one negative thing to say about the medical profession. A simpleton could understand this if not for the mind-control aspects of the cult.
4. Pray that your FIL and family will return to their right minds and the scales will be lifted from their eyes.

As a doctor and a Christian I want to say a lot of really harsh things about this sort of attitude, but that's not going to do anyone any good.

To give some perspective, plenty of people refuse medical care for all kinds of reasons, religious and non-religious. With the exception of public health concerns (like vaccines or TB) or sick children, that's okay. People get sick and people die. Some people we can help, some people we can't, and some people that we could help don't want our help. Sickness is unpleasant but universal. Death is just universal. It's going to happen sooner or later to everyone, and many people want as much control over that as possible. For some that means aggressive treatment, for others it means refusing care. There's no right answer as long as that person is at peach with their choices.

When it comes to cults, I don't think it does any good to be confrontational. They expect that and thrive off of it. That hostility helps keep them isolated from people outside the cult. IMHO, the best thing you can do is just let those people know that you care about them no matter what, and let them you that you're there if they need anything. Having a lifeline of support from someone outside the cult might be the thing that lets them get out.