on iPhone?

Does anyone use on iPhone?

I’d like to set it up for my girlfriend but I don’t even know if it’s possible.

The Cheogram app doesn’t seem to have an iOS port.

I found this app through the XMPP website: GitHub - tigase/siskin-im: (M) Public Project of Tigase Messenger for iOS devices based on Tigase Swift XMPP library.

Can anyone vouch for it? Anyone have a better suggestion?

You can use any xmpp app.
I use: Snikket Chat | Snikket iOS App

I use jmp with snikket on ios. snikket is actively developed and works great. jmp users get snikket hosting incl. in their jmp subscription -you can also self-host- its a nice integration imho.
jmp is out of beta and the subscription price was increased. however, you can pre-pay for >3 years and lock current pricing. -until end of august I guess-