install .ipa via iTunes on friends Mac

I don't want to erase all my existing apps. I just want to install the .ipa file and KEEP all my existing apps. However iTunes says

Are you sure you want to sync apps? All existing apps and their data on the iPhone "Simon Strandgaard's iPhone" will be replaced with apps from this iTunes library.

I really don't want to click the "Sync Apps" button.

I could go home and open iTunes on my own computer, but I'm at my friends place. How to do get the .ipa on my iPhone?

Solution 1:

You can use the iPhone Configuration Utility to side-load apps onto an iOS device without 'syncing' and erasing your current content.

Config Utility for Mac
Config Utility for Windows

Keep in mind, though, that this will only work if the app is correctly codesigned and the device is correctly provisioned.

Solution 2:

You don't. Apple doesn't enable you to install ipas separately. If its downloaded from the appstore you can re download it directly on the device. Also, you cannot install ipas that are connected to your friends account, as they containcryptographic keys unavailable to you if you never downloaded the app.

Solution 3:

If you have bought the app with your own iTunes account, you can simply open the App Store-App on your device, go to "Updates", choose "Purchased articles" and choose "Not On This iPhone". You can then pick the app from the list presented to you and touch the cloud icon to download.

It's not possible to sync an app that has been bought with a different iTunes account.