D”Y, holding hands with a niddah is NOT the same issur as eating at McDonald’s, and anyone who says so, is probably committing the same type of lo tosif as Adam Harishon did when he told Chava they could not even touch the eitz hadaas, whent hat was NOT what Hashem said to Adam.
The difference is is that Hashem DID say not to engage in behavior which may lead to giluy arayos.
but if they do, lightning will most likely not strike them
Not in this world. Same as a cheeseburger. Or as hello99 pointed out, worse.
they should not be told that they would get koreis for touching a niddah, if it simply is not true
Agreed, but they should be told that’s it’s a very severe issur D’oraisa, for which one one would be obligated to give up his/her life not to transgress.