iPhone 7 stuck on update 15.7.3 installed screen

My sister updated her iPhone 7 to 15.7.3 and now it is stuck on the confirmation box after the update was done.

Hitting OK does nothing, we cannot turn it off by holding the on/off-button.

Nothing behind this box can be accessed, afaik only the home button and side button(on of, dunno) work as they're told. Volume not too sure.Force restart does not work either, it just asks to swipe to turn off in the background and at some point the code is asked behind the update box.

The device is a 32GB variant and rather full on storage

Is there any other way than waiting for the battery to run flat, an Apple associate or third party software?

Any help is appreciated