You actually can transfer miles. Here is from the United Web site :
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New! Now you can share your miles with friends and family. For a limited time, transfer up to 60,000 Mileage PlusŪ Transfer Miles!
Give up to 15,000 miles to each recipient's account
Allow 7 business days for the miles to post to each recipient's account
Credit card will be billed immediately for transfer
Valid through December 31, 2005
Minimum transfer 5,000 miles per account
Frequently Asked Questions
Check order status after purchase
Mileage Plus Transfer Miles cost $0.01 per mile (includes applicable taxes) plus a $25.00 transaction fee.
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This will cost you 175$ for the maximum of 15000 miles. Not exactly a good deal in my opinion but if they are just short by a couple of tousand miles and already used up their other options like getting a Visa card or need the miles fast then this can be an option.
The same number of miles bought cost you as a gift $428.13.
The cheapest way is that you get a ticket for them. Just call United with the passengers name and have them search for award travel and tell them to deduct the miles from your account.