Can I update a friend's iPhone on my Windows PC iTunes?
Thank you for using Apple Support Communities!
We see from your post that you have some questions about updating a friends iPhone on your Windows PC. We have some info for you.
It's totally fine for your friend to update their phone on your PC. They don't have to sign into their Apple ID, they can just connect their iPhone via USB and update using the steps below:
Update software on iPhone, iPad, or iPod touch in iTunes on PC - Apple Support