There is no guarantee that the guy actually stole his BTC. Many wallet stealers won't take the funds right away but will wait until a sizeable amount is transferred into the wallet before it cleans it out.
This is what I am talking about. There is 90% chance that he did stole btc from OP, but still 10% chance that he did not. There is no strong evidence.
I would say even less than 90%.
Evidence supporting this person was the thief:
- He had unrestricted access to the computer.
- The funds were transferred almost immediately after he left.
Evidence against:
- As Blazr mentioned, wallet stealers often wait until a significant amount triggers them into action and this address hardly had more than half a bitcoin in it before the day in question.
- This person should have known that he would be the first suspected and that his victim could easily track him down.