[Q] OS from one phone to another topic

My son and I both have an LG G3. He's been playing with his phone and has flashed it with a different ROM. Now he is having issues and quite frankly I'm tired of it and want him to just return it to the way it was when we got them (We are in Australia and it had an Optus ROM).
We can't find the exact ROM online so I was wondering if it is possible to extract it from my phone and put it onto his?
I'm fairly advanced tech wise, not much Android specific knowledge, but with instructions I will be able to do pretty much anything (I work in IT and do Database coding) so not scared of some complexity etc.

Thanks in advance, been using XDA for advice and info for years, but never posted before.

Brendan