First, for the specific edit, I think the UK vs US spelling is not the main point.
In the post the spelling was mixed it said (my emphasis): "[...] should we block the capacity to favorite a question [...]" yet then "[...] lots of questions which have more favourites than [...]". To the best of my knowledge it is generally discouraged to mix the spelling conventions.
I would say if there is a mix for what is basically the same word it is simply an error (even if a minor one). I think it is generally agreed upon that minor spelling errors can be fixed, yet some restraint should be shown. (That is not mass-fixes of the same minor issue over the site. Also fix other errors in the post, not just one. Etc.) In that sense, I think the specific edit is fine.
For spelling more broadly, both are fine. There is no reason to copy-edit posts from one spelling convention to the other. Generally, copy-editing to fit some particular preferences for style is discouraged.
One additional point, for technical terms, such as "favorites" in the given context, there might be a case for giving preferences to the spelling actually used on the site. But that is a detail.