on your responsibilities?

If yes, I'll take out EVERY battery (even the back-up-battery)

on your responsibilities?

If yes, I'll take out EVERY battery (even the back-up-battery)

my responsibilities? no, I won't be the one to be blamed for anything you could possibly do wrong.  I can guarantee I wouldn't break something, but I don't know about your entire technical skills, and can't vouch for you.  I can vouch for the information and that I have screwed my calculator up with a bit more than what you are talking about (ie, no OS at all) and have been able to fix it.

If you've replaced the OS and nothing changed, then there are basically four possibilities: Boot code bug, Hardware bug, the hardware is broken, or User error.

I *highly* doubt  there's a boot code bug unless you've been playing with Flashy lately. If there was, your calc never would have booted from the factory.

Flashy is an app that mess up the boot code, but I don't think she played with it.
If there was a boot code bug, the calc would NEVER have worked.

so, in summary of what everyone has said so far, and laid out as clearly as i can:
-firstly, ensure you have a calculator of the same model as her calculator with you as well as a link cable that can connect the two
-secondly, remove the back of her calculator, take out one battery, hold the DEL key, and reinsert the battery while still holding the DEL key
-thirdly, press ON on her calculator. if everything has been successful it will show you the message:
Waiting...

Please install
operating
system now.
-fourthly, press ON, then another key when you see an error message, and then ON again to restart it. if all has gone well then you should see a Ram Cleared message.
-fifthly, if this does not solve the problem, repeat the DEL + Battery Insert step, but do not press on when you see the waiting message. instead, connect the two calculators via your link cable, on your calculator, navigate to the link screen (2nd + X,T,ө,N) and select the G:SendOS option. if you see an error message on her calculator then you most likely took too long and the sending has timed out. try it again. if it fails multiple times then it may mean that you are trying to send an incompatible OS from a different hardware model (if she has an 83+ make sure you have one as well. if she has an 84+ or 84+SE make sure you have one of those two as well, etc).
-sixthly, if you have started to send the OS properly you should see this message on her screen:
Receiving...
Operating
System
        [%received]
wait until the percentage reaches 100, you see Validating, and the calculator shuts itself off. this should solve the issue, and if it does not i don't have any further suggestions. good luck!

hmm, maybe something went very wrong and the batteries were drained, flooding the LCD?
anyways, it seems that her calculator is not accepting the operating system on your calculator as valid for being run on her machine. are you using the same model as she is? you aren't trying to send 2.55 to an 83+, are you?
if that's not it then i don't know what could be going wrong...
EDIT: i just decided to read the topic title and realised that the 'defect' calculator is an 84+. are you trying to send and operating system from an 84+ or 84+SE calculator to it?