JOEL - LDS believe that almost everyone will be saved into some level of Heaven after we die. However one needs to decide whether or not he is going to try to be saved into the highest kingdom in heaven where he can obtain eternal life with God. In order to do that we need to enter into the everlasting covenant of marriage; an ordinance that is only performed in our temples. There a man and woman are sealed together forever, and if they remain faithful to each other and to the promises they make in the temple, they will together obtain the highest glory of the Celestial kingdom in heaven. This of course requires that both are active faithful members of the church.
What those Mormons said to him was rude and only partially correct. No one knows what the future holds for any of us. Your former husband knew what he was doing. He was in love with you and perhaps he thought or hoped that someday you would also become a member so you could both be sealed in the temple together. Of course the ideal situation would have been if you were both members of the church. But there are many members who have married non-members and they can have a very happy marriage if they are tolerant of each others beliefs.