I guess I'm not clear on what "beyond our grasp" means to you. Does it mean that you can't even comprehend that it exists? Or does it mean that you can't wrap your mind around what it does, what its motivations are, etc?
Because, your parent seems to be saying the former (that we could comprehend that it exists, but wouldn't understand its motives, etc.).
But that's a non-statement. I don't think there's much question that we'd be able to perceive anything that our senses can detect. And, an ant can grasp that an object exists (and, for instance, that it needs to traverse it), even if it understands nothing about its purpose, who built it, etc.
So, it would seem that the only sensible intent in this statement by Musk (and the interpretation that you, but not your parent, seem to support) is that we would have a better understanding of this alien super-intelligence than an ant would of us, simply because we'd crossed some imaginary and arbitrary intelligence threshold, defined by Musk.
It's a comforting (and very human-centric) thought, but I don't think it's true. Considering "infinite intelligence", there is some point along the continuum wherein our relative intelligence to a super-intelligence is akin to an ant's to ours. So, I don't see any reason why we'd better understand the motives, etc. of such a super-intelligence than an ant would ours. To make that statement is to define the bounds of the super-intelligence based on ourselves and our own boundaries, which is to say that it involves completely circular reasoning.