Your spouse is your partner. This is your primary relationship. If someone you know is treating them poorly, and you choose to retain that friendship because, you know "gee, they've always been cool to me," then you're being grievously disloyal to the person who should matter more than anyone else in your life.
That's the thing though, re-read what the author said very carefully:
"that person is your enemy, even if that person has always been cool to you in the past, ... or your partner has never actually met the person."
If your spouse decides for any or no reason to hate your friend that they never even met, you're just an asshole if you then decide to mirror that sentiment just because.