No matter how much he tells them when the market goes down, they will always blame him. I have helped a long chain of my relatives and friends to dive into bitcoins and they always blame me when the market goes down. When things are going good, they feel proud and forget but if things go slightly wrong, they are all over me.
I am not too concerned about them though because I spread the word about bitcoin not just with a charity mindset but I know that by spreading about bitcoin, I am helping myself as well since I am an investor.
If he really warns them of the risk involved, they'll not come to blame him when things go wrong,
and if they try to blame him, he can simply remind them that there was a disclaimer.
But, before you bring anyone in, I'll strongly suggest that you are sure of what kind of person they are,
so you don't end up receiving constant disturbance. Just make sure they have a matured mind, that's just it.