Why exactly does this happen? Why is that order dangerous? And why is it better to start with the third eye?
You can put these facts up:
It's much safer to start upwards then end downwards due to the fact that the kundalini is in reverse order going from base to crown, meaning that if you have an open base chakra and blocked 2nd then the energy cannot go through the sacral and this generates heat.
I doubt anyone would awaken their Kundalini with new age practices, much less ascend it, however if one manages to do so and the other chakras are not open and energy is not able to circulate freely then Gods forbid what can happen, with pain being the least of what could be. When the kundalini ascends, it's supposed to be a very transformative experience, not one that sends you to the mental asylum and let's hope no one gets to know firsthand what delusions those who are without would experience as a consequence.
We are supposed to start from the feminine chakras then open the masculine ones; electric and magnetic - you could explain that they are differentials that complement the other side like Yin/Yang - dark having a small light circle which points that it shares Yang characters and vice versa.
Moreover, it's easy to open the uppers at the beginning and they help the person with meditation once opened. The Third Eye is an extension of the Sixth and thus it would appear logical that the 6th should then be opened.
You could mention chakra breathing exercises within the discussion, and see a way in which you can point to this. Appear wise with facts that support what you say.
If that does not work, then you can't convince the other party unless you come up with something. If they don't listen, then they are just ignorant of what is good.
I'm writing this while somewhat busy, so excuse me if I haven't been able to put other arguments in. Hope you will succeed.