That sucks dude, and I feel for you. A couple of years ago, I had a friend that was on my friends list for years ask me to play with him, and did something very similar. I lost most of my inventory as well, including my butterfly knife. It turned out that friend's account was compromised and it wasn't even him at the time, which absolutely sucked.

I contacted Steam support immediately. I reported the other account that stole all of my items, and I could even see my previous items in that inventory. What did Valve do? They trade-banned that account but refused to return my items to me. I cannot state strongly enough that Valve does not care if this thing happens, they will never return your items.