How do I find a friends iphone not listed on my device?

Okay.

These are the steps you can perform:--> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Regards

Giulio

If it isn't among your contacts, you have two possibilities:

If you had a recent conversation with that friend, check your recent calls history.

If you have her/his email address, send her/him an email message asking for the phone number.

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

Regards

Giulio

My pleasure