Q: How to unlock iphone without personal sim?

Hello, you need to have a sim card in order to activate your iPhone. See more information here If you can't activate your iPhone - Apple Support

Borrow an appropriate sim from a friend/relative/co-worker

for the few minutes needed to complete activation. You will

need a sim to update the iOS version or restore the iPhone.

Once activated, the iPhone can be used as an iPod without

the sim.

I'm not sure what you mean by a "personal SIM." The SIM contains information about your account with your carrier, including your phone number. You can just go to your carrier and they will give you a SIM. They'll even set up the phone for you.

A separate question is whether the phone is activation locked to the previous owner. If it is you will need their cooperation to make the phone useable: Find My iPhone Activation Lock: Removing a device from a previous owner’s account

You can't use an iPhone 4 SIM without cutting it down; then he can't use it. So try to borrow a SIM from a 5 or later model. Or go out and buy a Pay as you Go SIM. They are available everywhere.

It won't mess up her phone. iPhones do not store contacts on the SIM card. While it is in your phone it will be "her" phone as far as the carrier is concerned; any calls will go to it. But you only need the SIM for about 5 minutes.

Yes. Just be careful and don't lose it. They are tiny.