Said or (if they did not necessarily use those exact words) thought are much better. Opined (unlike the noun opinion) is an unusual, literary word, and in this context it suggests that you are being sarcastic - maybe you are suggesting that their opinion is bullshit.
A lot of people don't like opine as it is a back-formation from opinion. I remember using it in a literary essay when I was an undergraduate, and my lecturer circled the word and said it was a 'vile villain of a word'. Even literary people don't like it. Stick with said. Alternatively agreed would work too.
Some time ago I did a search on the verb(s) formed with 'opinion' in different languages.
Besides 'to opine', there's also 'to opinion' and 'to opinionate'.. I don't know if those are of 'normal use' either. .
I like to use 'to opine' and OED backs it.
However, there has been a row over a John Cleese opinion where he is using 'to opine'
<Some years ago I opined that London was not really an English city any more>
and there was a one reaction saying:
<...it is basically a very racist tweet. Secondly, it’s a racist tweet by a man who lives on a Caribbean island – the irony of that is insane. Even the language is insane. Who uses the word ‘opine’?” >