I am so sad for their family.
I am thankful that I knew her. I am thankful that I worked with her and was in one of the best wards in the world where we still keep in touch, still send letters and Christmas cards, still mourn with each other... I am truly thankful for those associations.
As someone who has lost both parents at an early age, I can tell you that little anecdotes about your parent's life, those little remembrances told by people that loved them or worked with them or just associated with them somehow are hugely comforting. Not just the "you'll see them again" comments which are nice but I feel are a little trite. Mainly because most Christians believe in life after death of some sort so we want to hear something more personal in these moments.
Remember that although intellectually they may believe in eternity and life everlasting, the heart is broken and tends to rule. It's ok to cry, it's ok to be angry, it's ok to be sad. Don't try to force the healing process by saying how wonderful their reunion will be some day. We know that. It may just take a little time for that to comfort us. Just share a nice moment in time about their loved one, something that will make them proud of their family, and be a friend. And all will be well eventually.