install .ipa via iTunes on friends Mac

Question

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?