I don't believe it is against eBay's policies to bid in friends' auctions.

I believe it is only "family members [also boyfriends, girlfriends], roommates and employees of sellers" who can't bid in your auctions. Friends are not mentioned. (Of course, anyone --regardless of the relation-- is not allowed to shill. I am only referring to mere bidding, in this post.)

Look here:
http://pages.ebay.com/help/policies/seller-shill-bidding.html

The full rules are as follows. If you know of other rules, let us know. eBay rules:

Shill Bidding is bidding that artificially increases an item’s price or apparent desirability, or bidding by individuals with a level of access to the seller’s item information not available to the general Community. Shill Bidding is prohibited on eBay.

Because family members, roommates and employees of sellers have a level of access to item information which is not available to the general Community, they are not permitted to bid on items offered by the seller -- even if their sole intent is to purchase the item. Family members, roommates or employees may purchase items from a seller without violating this policy simply by using purchase options -- such as Buy It Now -- which do not involve bidding.

eBay strongly encourages sellers with employees to ensure that their employees are aware of this policy and the possible consequences of violation.

Violations of this policy may result in a range of actions, including:

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Listing cancellation

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Limits on account privileges

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Account suspension

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Forfeit of eBay fees on cancelled listings

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Loss of PowerSeller status

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Referral to Law Enforcement

Spend a few minutes taking eBay's tutorial on Shill bidding.

If you believe that another member is artificially raising the price or desirability of an item by shill bidding please report it to eBay. Please be sure to provide the user ID's and the item numbers in question.
Some Examples:

Below are several examples. In each of these examples, Shill Bidding has taken place. Shill Bidding can take place in other situations than those illustrated here: remember, the key issues determining Shill Bidding are manipulation of price or desirability, or bidding by individuals with a level of access to the seller’s item information not available to the general Community.

Jean operates a very successful business auctioning items on eBay. Ben, an employee of Jean's, wants to purchase several of the items offered by Jean and places bids on those items. Even though Ben really wants to buy the items, his special access to information not available to the general Community creates an inherent, unfair advantage.

John is selling his extra MP3 player on eBay. During the auction John’s girlfriend, Jane, places a bid on the MP3 player even though she does not intend to buy the MP3 player. The MP3 player is won by a third person, Bob, whose bid is $5 higher than Jane’s.

Sally is selling her car on eBay. During the auction, Sally becomes worried that her car might sell for less than the amount she is hoping to receive. To ensure that no bidder can win her car for less than the price she has in mind, Sally uses another eBay account to place bids on her own car, increasing the volume of bidding and raising the price to the level she feels is ‘fair’.

Bob is a high-volume seller on eBay, who wants to be sure that he never sells at a loss. Rather than use eBay’s “Reserve Price” feature to set his minimum sale prices, Bob has an associate place bids on his auctions... in effect, setting a ‘hidden’ reserve price.

Robert