What shavings are harmful to hamsters, rats, bunnies etc?

Pine and cedar are harmful. Aspen is good. Here is an article.

There’s also a bedding called Carefresh, made from recycled paper, which is safe for small animals.

Aspen is the best shaving – don’t put pine in there, it causes respiratory issues. The recycled paper is fantastic, a little more expensive but the ultimate choice.

Has your brother considered getting 2x rats? Rats are naturally very social animals and group in family units. One rat will be lonely and depressed even with large amounts of human interaction.

Two rats are no additional work to care for, trust me.

PM if you would like rat care advice, I have had rats for years and a prominent rat club member.

Paper-based cat litter is the best for rats, and he should not be keeping one rat on its own. He should get another (of the same sex, obviously). Rats are highly sociable creatures and need company of their own kind.

Bed A Beast is harmful to all animals. They tend to eat it and die from bowel impaction. The best bedding I have found for any caged animal is rabbit pellets (feed). They are just pure alfalfa and full of chlorophyll. They are absorbent, and they kill odors. If your pet decides to eat them, it won’t hurt anything! They are also quite a bit cheaper than most bedding. You can get a 25 lb bag of inexpensive rabbit pellets at a feed store for $6 or $7.
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Here is an article I wrote about pet housing:
Housing Your Small Pet for Less
http://www.associatedcontent.com/article/1921886/housing_your_small_pet_for_less.html?cat=53
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I hope it will be helpful to you! :)