ANSWER:

Don’t do anything!

You both are sinful for spying on your father in the first place, as this is a major sin in Islam. Shame on you trying to judge him and parenting him as if he is the child and not your father! 

QUESTION: Sheikh my younger brother accidentally discovered that our father watches p@rn , he showed me but none of us ever saw him watching but it’s in his phone’s history. Our dad is mostly out of the country and comes for a few months he also prays his salah . After seeing that I am a little worried and confused & all sorts of mixed emotions I don’t know what I do