I once asked my Rabbi whether it was permissible to pray that a person pass away so that they not suffer and he said that is not correct to do so. He said that the right way to phrase the prayer is that the person not suffer any more without defining what that means. The prayer is ambiguous enough that it can mean either that the person be cured of their suffering or that they pass from this world and not suffer anymore. Even if that is the inference it is permissible to pray that they not suffer as you are not asking that their life be shortened.
