I absolutely think that happy, healthy mono-poly relationships exist. Even within polyamorous relationships, there are situations where one partner might choose to date and the other might not want to date others at that time. There are lots of reasons for that, time, emotional state, work and family commitments etc. As long as both people are happy with the arrangement and having their needs met, it’s not necessary to compare one to one or you have partner, I need one too.
Regarding your friend, there’s no way I can make a judgement about her particular relationship.I don’t know what your friend’s relationship agreements or arrangements with her husband look like. If she is doing all of those things with his blessing, it sounds like it could potentially be a mono-poly relationship.
That being said, there’s not enough information here to know if that’s what’s going on. If she prohibits her husband from seeing other people, but expects him to be okay with her doing it, that is potentially unhealthy. If she is doing all of this behind his back without him knowing about it, that’s not polyamory at all, it sounds like it could be deception/cheating.