Pat, there are no cut and dried rules of thumb when it comes to frost damage. There are too many variables. Some are hurt more easily than others. Once you have frost damage, it becomes a game of 'wait and see' what happens. Some will continue to grow up through the damage, some won't. Here's a tip for you since I know you like regales a lot. They are very susceptible to frost damage and one of the first to suffer should there be the slightest of frost. Part of that is because their leaves are so narrow. So, if frost is predicted, those would be the first ones to get covered.
What your lily breeder friend told you is basically correct, but if the damage is severe enough you may see some buds abort and drop off when they reach bud stage..
