A friend gave me a phone and they forgot to remove password, how to unlock?

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

If you are talking about the passcode to lock/unlock that iPhone's screen:

Keeping in mind that if you don't have a backup all data will be lost on that device (except those that you eventually synced to iCloud), pay attention to the specific gestures that you need to perform while connecting your iPhone to a computer (Windows 10 PC running the latest iTunes version) or Mac (which can have iTunes or Finder, depending on the macOS installed on it), and follow these instructions to re-gain access to your device:--> If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

or watch this support video about the same matter:

https://www.youtube.com/watch?v=7s8-zLG9ySQ

If you mean something different, please better specify your needs.

Regards

Giulio