I just lent MW to a friend, GOTY edition on 360. He just did the Fargoth ring quest, but now when he equips the ring, it does not show up on the screen when he leaves the menu. It's just a black square instead. And he can't go into magic mode either. He hits Y but nothing happens.

Any ideas?

Equipping a magical item does not place it in the Magic slot.
You have to enter your menu, and select the ring in the Magic Menu. (the text list of all your Spells/Scrolls/magical items)

You do the same thing for Spells and Scrolls that you want to use/cast.

The black square just means that nothing is selected, and you cannot ready your hands to cast unless something is selected there.

Good answer Pluto, I believe this is the case.