I do not know the name of the product but Long's drug store has a dry shampoo.  It's used for people who are bed ridden or whatever else may prevent you from using water.

It is in there regular shampoo section ...g

I have heard about dry shampoo...I dont know where though, sorry! If she is worried about looking greasy, she could try baby powder. I have also heard about people using corn starch...I like baby powder though, because it made my hair smell nice.

Also I've found that now that my hair is black rather than blonde, I cannot add powder to it because it makes it look very dull and kind of weird. Just something to be aware of.

I know a few people who have used cornstarch as a dry shampoo.  They sprinkle a bit on the hair, massage it a bit so that it can absorb the oils, then brush out.

I've not done this myself, so I'd recommend trying it at home before planning on this as the primary option on a long trip.