I would suggest they try to imagine a white light surrounding them before they retire for the night. inside this white light they should see themselves being protected, and nothing evil will be able to penetrate this white light to get to them.
as for crystals, I'm not really up on all my crystals and herbs to know which are great for protection like this, but there are many threads regarding this very topic. I have just read by googling 'crystals for protection' that bloodstone is a good protector, as well as the following:
smoky quartz
turquoise
black tourmaline which is supposedly excellent to use as protector while you are asleep.
amethyst is another good crystal for protection.
you can either keep one of these or many of these stones near your bed, or under your pillow or wear it as jewelry.
as for the herbs--again, via good ol' google, agrimony (which I've never heard of before) is supposedly a great protective herb. as is angelica. this i had heard of before, but had forgotten till i found it again on google. you can plant these herbs in pots or around your home for the protection, or I imagine just having them near you as a dried herb will work too.
another herb I had always heard of using to help ward off evil and prevent nightmares was anise. now, i cannot recall what it tastes like, but i think it is similar to black licorice, which i never could stand. this herb is also great for relieving stomach cramps, (esp. menstrual) and toothaces. basil is yet another herb that does the same as anise--and THAT I can stomach much better! we have a basil plant in our home, and keep it all year long. you can generally find these plants, roots and all in your grocery store, in the vegetable section. buy it, and plant it in a pot, and keep it in a sunny window. easy to grow and very tasty too! it'll also help with bringing you wealth!
good luck to them, and I hope they soon find some relief.