I need to mark a lost iPhone

Here are the instructions. Hopefully, your friend enabled Find My on her phone....before....it was lost.

Find your lost iPhone or iPad with Find My - Apple Support

Please keep in mind that even if Find My was enabled on the phone, it will not show up on the Find My map....IF.....

The battery has run down

The phone has been powered off

The phone does not have an Internet connection where it is located

Finally, watch out for scam messages from "Apple Support" or the "Police" that say that the phone has been found and they just need a bit more information to return the phone. These are SCAMS trying to trick the user into providing their Apple ID and Password. Never reply to a message like this.

The phone is already locked so the bad buys cannot get into it.

To log on and use Find My, you need her Apple ID and Apple ID Password

Apple ID is her email address. If she has forgotten the Apple ID password.....If you forgot your Apple ID password - Apple Support

I'm afraid that any chances of ever finding the phone online are gone if the correct Apple ID and Apple ID Password are not entered when you log on to www.icloud.com/find

We've done all that we can do to help. Good luck.