I generally have bonded pairs of cats, and they do grieve when they lose their partner. Usually by searching for the departed cat, not eating, crying, and being very clingy.
With Ziggy however, I did not expect deep grieving because Mollie was still with us, and he was always so independent.
But he went into a deep depression and hid away for about ten days. He came out in the middle of the night to eat and use the sandbox. The rest of the time he hid away from us all at the back of a shelf in the darkest depths of our walk-in closet.
He finally came out of hiding this last weekend. He came for cuddles, and has been out in the open since then. We are giving him as much love as he wants. Welcome back boy!