For baking savory items, use unsweetened soy milk. If baking sweets, Lactaid milk works great; it tastes sweet, so I usually cut back on the sugar a bit. If the recipe calls for buttermilk, add 1 TB of vinegar per cup of either milk mentioned above. Depending on the recipe, coconut milk works too. Adds another flavor dimension.

It definitely depends on what you're making, but generally you can substitute soymilk for regular milk in most things without a problem. Most soymilk is sweetened, so watch how much sugar you're adding or things will turn out sweeter than you meant. If you're making something savory, try to use unsweetened soymilk, or at least a plain variety; don't use vanilla-flavored soymilk as it tends to be extra sweet.

See my recipe for cranberry bread here. It's gluten-free and dairy-free and I use plain soymilk in it:
http://beyondcelery.blogspot.com/2010/10/time-for-cranberry-bread.html

Quiche or other custard-like fillings can usually be made with soymilk. They tend to turn out better with the addition of an extra egg to help it set up. I add cornstarch (about 1-3tsp) to homemade pudding and it sets just fine with soymilk.

I've used rice or soy milk in all my baking for ages and they work just fine. If she's allergic you may have to use vegetable shortening instead of butter as well.

You could use Lactaid milk instead of regular milk. You can't really tell the difference, especially in baked goods.

Depends on what you are baking. In general though, lactose-intolerant people can usually drink soy milk, almond milk, rice milk, etc.

You should check out The Divvies Bakery Cookbook for great allergen-free baking ideas: http://us.macmillan.com/thedivviesbakerycookbook