Transfer app to another account?

You cannot. iPhone Apps are DRM-protected and linked to the account used to download them. The only way someone else can use the app is if they are using your iTunes Store account.

Just copy that app to your father's PC and authorize his PC with your iTunes ID

Apps are DRM protected & tied to the account that was used to purchase them. They cannot be transferred to another account. However, you are permitted under the EUSLA to "share" apps with up to 5 users in the same household. To do so, you would first disable auto sync when an ipod/iphone is connected under preferences, which in windows is under the edit menu, on the computer in the same household you want to share this app with. Connect your phone, itunes running, DO NOT SYNC!. Sign whoever's computer this is out of itunes.. Sign into itunes using YOUR ACCOUNT INFO. Go up to Store>Authorize This Computer. Then go up to File>Transfer Purchases. All of the purchased content on your phone will be transferred. Sign out of itunes, disconnect your phone. Sign whoever's computer this is back into itunes. You can then sync the app in question to their phone. All updates to this app must be done through YOUR account.

You can do the same through itunes "Home Sharing" feature.