TheUnforgiven - that could totally happen but at a bare minimum would incur several million in fees, for both Tencent and Sweeney, assuming both parties wanted to do the transaction.
Probably much more material would be the tax position of both Tencent and Sweeny's company. Tencent's value of EPIC shares has increased enormously, and I would have thought would incur a huge taxable profit if they had to be sold. I know nothing about chineese tax law but i'd assume that any large increase in realised investment income would be taxable. If they don't sell the shares they don't have to pay the tax (in UK and US rules).
Similarly Sweeney's private arrangements would need to raise the money to buy the shares at their new, much higher value. That's a lot of financing to arrange and you need to find investors willing to buy.
riotous I think you're right, I just wanted to make it clear that the situation is pretty complex. Sweeney may be able to do what he wants but then the other Board members might make it difficult for him to do other things he wants to do.
I did a quick google and Polygon stated that Tencent got to appoint two directors to the Board, with their initial investment. I also found the Tencent report which mentions it for YE2012.
I can't quickly find any details on Epic's board of directors so I can't see how many there are in total, or who sits on which committee. They're not listed so it's all pretty opaque. The joys of giant private corporations!