it is an excellent question and to be honest I have my doubts about a western response other than outrage and finger pointing. Because there are only three responses-one is NATO joins in in full for conventional warfare which would make the Russians go strategic nuclear anyway.

Or the US and its allies state "use of nukes is use of nukes" and launch strategic missiles.

Or...the US and Europe decide that the use of a tactical nuclear weapon by Russia does not require the whole world to suicide itself(except for some mad prepper in a bunker somewhere saying "haha I knew it").

My bet would be on the third option and Russia guessing that option would be used which would encourage them more to do it....but then again...not in a million years would i have expected such a united response from Europe and the US in assisting Ukraine. So nothing could be ruled out.