I agree with the PP. And I would start a Word Doc and list dates, times and details. Just like your post on this page with specifics like:
"10am, I was given 2 projects and T was given 2. By 11, I had done x, y, z and T hadn't come to ask me for her part, so I walked them over to her desk and put those 3 items (x, y, z) on her desk. At noon, I was given 3 more. I did them. Still didn't see T. Walked those 3 over to T's desk. Observed the 6 I'd given her that day sitting on her desk while she was on otherdude's desk chatting with him." etc...
"By closing time, I had finished 10 projects. By then, I had given 10 assignments to T. At closing time, I received an email saying T needed help with 10 projects. I imagine that since I was the one who gave them to her, those are the 10 I had put there. Which means that all day she just spent her time at dude's desk talking to him."
You might leave the conjecture out. But I'd keep a running list of these things. Every single day. Hopefully, the paper trail will help your boss. If you trust other coworkers, suggest they do the same.