As you must know, your WM ID is not the main thing when you pay, but WM purse of the billing and the NOTE (or Comment) to the payment.
My billing login is suma and I have to specify my billing login in the comment to the payment: suma deposit. Payment will be deposited within 5 minutes (read comment to Webmoney option of your billing).
So, enter your billing page, select the option "add funds", specify WM purse of the billing and billing login of your friend. Check within 5 minutes if your friend's account is deposited successfully.