That's just the rubber, not the actual focus ring. It's a cheap consumable/replaceable part. Just look for the part number, buy it from camera parts shop/ebay/whatever, tear the old one off and slip the new one on.
Something like this, for $19:
But that's for 24-70 f4 IS, if you search for it there must be one for 24-70 II
Thanks for the tip. However I'm not paying for this, neither installing it myself, and not with a non-original part