That is horrible! How would this person know what the reason was. I think I would have had a much better answer for her. One, man cannot always understand the ways of God. Afterall, we are just humans and HE is God afterall. Or highest thoughts are like foolishness compared to those of God.
We can know the character of God. That he is Love, That HE is faithful, forgiving, caring, compassionate etc. etc. I could go on. but I think you understand what I am trying to say.
If you understand that we live in a fallen world, because of sin entering the world, then you would understand that all of creation suffers because of it. We no longer live in the Garden of Eden where it was perfection practically on earth. We see death as something horrible. Jesus had compassion on the woman whose son had died. He understood her sorrow. But in the grander scheme of things, our life is but a breath compared to eternity. There is a verse that says that "precious in the eyes of the Lord is the death of His ( or the) saints." That is because HE knows that they are in paradise for eternity. How glorious is that? Well, we won't really know until we get there! But it is going to be fabulous beyond what we can even begin to imagine or fathom.
We see things only from this side of Glory. It is hard to comprehend. It would maybe be like trying to describe to a blind person what the world looks like and for them to be able to comprehend it. They wouldn't be able to 'get it' unless their sight was restored. That is where faith comes in. WE base that on the character of God and knowing that even when we cannot see what He is doing (or understand it) that we can trust the loving heart of God - always!