Will she be staying in the house or just coming to feed the pets and lock up.

If she is staying then i'd say you cover foods costs and possibly any extra travel expenses for the 5 days and a nice bottle of wine.
If she is just coming and going then maybe a few $ for petrol and a nice box of choccy's and alcohol of her choice.

But i guess it depends on the friend, we would do it for friends for free, if we had to stay then possibly get them to chip in for petrol if it meant extra travel but otherwise it would be out of friendship.