Will data be lost in resetting macOS admin password? My GF recently started working with a Mac and while I'm a IT tech with Linux and Windows knowledge, I'm very much a noob with Macs.

The problem is she's working on a Mac that another user (the guy she replaced) set a admin password, now no one knows said password neither does he pick up the phone.

I've seen there’s a way to create a new admin account via this post:
How can I get admin access to a Mac without knowing the current password?

My question is will she loose any data from the old account? She has some projects and such (I know the keychain will be lost, but she has all the passwords she needs).
As you mentioned, credentials will be lost including the Keychain, but files will be preserved. You can either create a new account, log into it and change the permissions on the original account’s home folder to be able to access the files, or reset the password to the existing administrator account and log into that. The former will preserve the Keychain in case the password is recovered while still giving access to the files.