How worn out is this tire? (Street use only)?

Keep in mind that this picture has flash on, so all the small details might look a little worse than what you might normally see. So long story short I asked a mechanic about the tire he said it should be changed immediately, my dad asked a mechanic from Honda and he said it still has like 400 miles to go. Last night the rear tire slipped on me for the first time (I was breaking at about 37 miles an hour semi hard and there was grains of sand (powder like) on the road. So basically The rear tire slid for about 15 yards until I hit traction and safely stopped. I've been wondering if this was just my fault or just the tire. I'm thinking that it was both, but that the tire really does need to be changed immediately. However I'm a new rider so I want to know what you all think about this tire and whether it needs to be changed yet or not?How worn out is this tire Street use only

Few tires WON'T slip on fine powdered sand.
Your tire pattern is designed more to wick away water, and on most tires, the more original tread is on the tire the better.
"Slicks" like they put on dragsters are softer rubber, and they "burn out" just before a run to make them "stickier" for traction.
Your tire may still be serviceable, but it is hard to know for sure from your picture. Compare it to a new one and see the difference first hand. If yours is borderline, replace it if you can. Trying to get the very last bit of wear out of a tire is not always the best plan.
Not sure, but it looks like your DRIVE CHAIN is a little DRY. When did you lube it last? Won't hurt to check.