I think the person who replied was making the point that for someone to be successful, others have to fail. This failure is obviously not an object of rejoicing. Since all success and failure comes from previous actions, we can rejoice in the successes of those who have created the cause, whilst understanding that such success is very fragile and momentary, and pray for their ultimate success of enlightenment. Where there is failure, we can pray that those people purify their negative karma, achieve success in the future and finally achieve the ultimate success of enlightenment.
There's always a positive and virtuous way of looking at everything.