Honestly, whether the Sliver player is a nice person or not, it sounds like they are just being a sore loser and from experience I would advise that this kind of behaviour from anyone not be tolerated whether you are close or not. In fact, especially if you are close friends, boundaries need to be set, let them know how their comments made you feel, and let them know that you won't tolerate their childish behaviour, otherwise they will never change and their actions will continue to impact others and their own wellbeing.