How do I find my moms iPhone if we have separate accounts?

Use any internet browser and log into www.icloud.com/find with her Apple ID and password.

Also see: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

You log in to her account at https://icloud.com and use the Find My app there to locate her device.

Or if she shares her location with you, she should show up in the People tab in the Find My App on your devices.

Link-> Share locations with family members and locate their lost devices on iPhone - Apple Support