She already spoke with the parents and they say that she should not mix in; what to do then?

Listen carefully. When your parents explicitly tell you not to interfere, you've reached a very difficult but clear boundary in halacha. You've done your obligation by speaking up - you tried to protect them, you spoke with respect, and you attempted to help them see the reality of their situation. That was the right thing to do, and Hashem sees that you fulfilled your responsibility. Now that they've told you not to interfere, you must respect that boundary. Why? Because continuing to push against their explicit wishes would actually become disrespectful and violate kibud av va'em. The mitzvah of honoring parents means that once they've made their position clear - even if it's a foolish position - you cannot keep forcing the issue. You've warned them, you've tried to guide them, and now you must step back. However, this doesn't mean you abandon them completely. You still have obligations: First, you must continue to honor them in all other ways - help them with their needs, speak to them respectfully, be there for them. Second, if their situation becomes truly dire - if they're literally going without food or medicine because of these decisions - then you may need to act again, because at that point it becomes a matter of pikuach nefesh, saving their lives. Third, you should daven for them. Make it part of your daily prayers that Hashem should open their eyes and protect them from the consequences of these bad decisions. Sometimes when we can't change a situation directly, Hashem wants us to turn to Him in prayer, and that prayer can accomplish what our words cannot. Remember, you're not responsible for their choices - you're only responsible for trying to help them and for treating them with honor. You did that. Now you must trust in Hashem that He will watch over them, even when they're making mistakes.