Your friend should contact the seller straight away and ask for the battery, which would have to be posted at the seller's expense if they just forgot to put it in. If you get no joy in a very few days, your friend should open a dispute about the item on the ebay page. If eBay uphold the problem and the seller doesn't resolve it, eBay may be able to refund you but you have to go through that process. A decent seller won't want bad feedback so will sort it out for your friend.