How can I find a lost iPhone if I don't remember the Apple ID?

You will need to gain access to the account by resetting the Password first. There are only 3 ways to reset the Password depending on if you have another Trusted Device, or Trusted Phone Number that was set up on the Account. If you do not have those 2, then you will have to use the last option by using the iforgot website. This option may take some time to verify your identity.

If you forgot your Apple ID password - Apple Support

Without the AppleID and password, he’s limited to friends and family with whom he might have shared his phone’s position.

See this article for possible alternatives to regain account access.

If you forgot your Apple ID password - Apple Support

If you forgot your Apple ID password - Apple Support