One might find it incorrect or impolite, or other way not good to bid against own friends. This all reminds me boxers friends which would not hit each other at the ring because they are friends, but usually they do hit as the sport winer is the one same as on eBay the auction winer is one (even for dutch auction,there cannot be more of winers than items offered).
So what to do if You found the item and want it, but You clearly know that Your friend has put a bid on it?
Possibility Nr.1 - Find the similar item and win it
Possibility Nr.2 - If there is no chance to find similar item to which Your friend is bidding to and You certainly know that You need it,but wish not to bid against your friend, You may ask another friend of yours, who is not a friend of Your bidding-friend to bid against him.
Possibility Nr.3 - Create another eBay ID (eBay legally permits to own eBay ID's more than one for one person),or ask some of Your relatives or friends to create this eBay ID to make the bid.
Possibility Nr.4 - Bid with sniper program and later blame sniper program. (Why should you have called it back? You were the first to see the item and You used e-sniping).
Possibility Nr.5 - Ask Your friend a permission to bid against him. (even that could be effective).
Possibility Nr.6 - No not bid against Your friend and forget the case.
Advice Nr.1 - If You see the item You wish and You expect it can be found by Your friend to bid at - Bid immediately in hope that he would not bid against You.
Advice Nr.2 - When bidding,make sure that all above mentioned possibilities can be used by Your friends against You, and if You happen to be outbid by them do not be jealous or angry if they are friends indeed!

Practice shows that this topic affects negations and that is why several people voted against.
Please vote for the guide as useful or useless!