My cat, Sterling (3 yrs in April), keeps jumping on kitchen counter and tearing into any human food

It was not always like this. In fact until six months ago he never even had an interest in being on counters or human food. We had never given him anything but cat food. 

6 months ago we went out of town to visit relatives leaving a friend to care for our cat. We were gone for 5 days.Suddenly we had a cat that jumped on counters and wanted human foods. We had given our friend permission to eat at our home and encouraged her to play with Sterling so he would not get lonely. When asked if she fed him or left dirty dishes out over night or anything of the sort, she got defensive and refused to talk about it.

Sterling is my emotional support animal. I love him so much and he is a wonderful cat who comes to cheer me up and be a lap kitty whenever I am upset. He is playful and this really truly is the only bad behavior I have seen in the 2 years I have had him. 

Today he tried to eat a pack of chocolate while we were at the store. Luckily he did not get through the wrapper. I am worried he will make himself sick or worse. We have trained him to sit, get down, stay. He is a smart cat. Spraying him with water hardly works, We got him a tree in case he simply wanted to climb. He only does this when we are gone from the house. He gets cat food through out the day (9, 12, 6, 9) and treats/training around 3. 

What do I do? How can I fix this?

Hi Cimerone and welcome to TCS!

Water spraying really isn't useful.  If you're concerned about him getting up on counters there are a few things to try here:  [article="22407"]Get Off How To Discourage Your Cat From Jumping On Counters​[/article]  

Otherwise I'm thinking it is best to just reinstate your old routine.  There is a slight chance (and I hope it isn't the case) that he many not have been fed properly or enough and started to scavenge.   Whatever has caused it, I think you being home giving him love and keeping to a strict routine will do the trick. You obviously love him very much.  

 When I have people looking after my two I leave 'War and Peace' length instructions   I really suggest you do that in future if the cat sitter doesn't know a lot about cats, and that seems like the case with this person. 

The only way you can fix this is to never give him access to food again, over time he will forget but it does take time.

5 days won't stick for his lifetime, but he was clearly fed while you were away, which allowed him to taste what people food is like, and he got hooked.

Just stay on top of it always, anything you don';t want in the fridge, put it in the oven or microwave.

I have to do this at my house too, my son in laws cat is here while he is in the army, and is ruined for life, as he was fed people food since he was a kitten, and is extremely obsessed, it makes me so mad!