Frost forms when a surface cools to a temperature which is colder than
the dewpoint of the air next to the surface. The frosted area of the
roof must be cooler than the rest of the roof and has nothing to do
with moisture in the roof itself.

That does make sense, but why are the shingles wet when it's not below
freezing? That's why I was thinking that it's this wetness that causes
the frost.

You all have some very interesting theories! My neighbor hasn't called
anybody yet to look at her house because of the holidays, but I hope
she will soon. She already had a leak in her roof in another spot last
year. She usually calls the guy who built the house, but I would think
this is not a good idea. If anything was in fact wrong with the
construction of the house, he sure wouldn't admit to it and fix it, he
would try to cover it up instead.

Glad it ain't my house!
Angie