I bought an iPhone 11 pro from Verizon, can my girlfriend who is on AT&T just pop in her sim card and use it on the AT&T network?
Have you had the phone activated on the Verizon Wireless network for at least 60 days? If not, the phone might be still locked to VZW. See the Device Unlocking Policy for more information.
https://www.verizon.com/about/consumer-safety/device-unlocking-policy
