What Not to Say" is just as ridiculous.
"This is God's will" is another attempt at compassion gone horribly wrong. Along with "God needed another angel", these trite phrases are an attempt to explain the unexplainable, to make sense of something senseless, and to apply reason to something that is completely unreasonable.
Theologically speaking, we cannot, in any way ascertain God's will, whoever or whatever that god might be. People who might self-identify as "Bible believing"Christians will tell you that reading the Bible will help you ascertain God's will, but this is simply not true. Because the Bible is a composite document, there is not one cohesive picture of God or of God's will. Furthermore, there is not a way to provide an answer in advance for every single contingency that might arise. The best thing that the Bible has for us to figure out God's will is a very rough algorithm. We hear over and over again about justice and compassion and care for others as being God's will, so let's go with that.
Which is why this particular platitude is so very asinine. Because telling people that their loved one's suffering or death is "God's will" is just about the opposite of justice and compassion and care for others. Like most of these sayings, this one probably starts from a place of desiring to offer compassion, but the best intentions get lost in creating an image of a God who plucks people out of families and lives at will.
I have witnessed deaths of all sorts, those from traumas and cancer and violence as well as old age, and NO DEATH is God's will. Death is a biological inevitability, it happens to all of us, and trying to blame a particularly tragic death on God's will just does not make sense. It is God's will that we would love one another.
So therefore, what to say? How about…
I love you
I care about you
I am sorry
Can I bring you dinner?
Can I watch your kids for you?
Can I walk your dog for you?