I have a similar situation with my mum. However, to preserve the relationship with her, I need to let her have her belief. We know it is a high control cult, however, some people are heavily invested in this cult.
Although it is a loving and caring thing to try and warn a loved one that this cult is dangerous, this person needs to have doubts, you cannot simply "implant" these doubts into an individual, the mind control and years of indoctrination is far to strong.
Sadly, I don't think my mother will ever wake up, so me trying to wake her up will NEVER happen. Sometimes we have to learn acceptance and focus on the cult itself. What I mean by focusing on the cult is knowing that this is not the cult victim's fault, it is the cult that slowly manipulated the victim into total servitude.