So essentially you can invest with perfect hindsight. The counter party, Aviva, didn't write the original contact but acquired the company that did. They haven't paid out any money yet and have contested the contract many times in court, loosing each time.
It's gotten worse for Aviva as their original plan was to delay paying out to bleed him dry to make him settle, but a few Swiss banks have lent him money to live off of and lent even more so he can leaver up his bet as he's allowed to invest additional funds into this contract.
The biggest issue is that you really can't value the contract as the holder is 25 and it pays out until his death. If he lives until he's 75 its worth more than the company could reasonably find.
If you are wondering why the insurance company would write such a contract, it was originally designed to be like a savings vehicle back in the 80s. It would let you swap between bonds and equities as your risk profile changed over the course of your life.
Back in the 80's it wasn't really considered feasible to sort through all the markets and figure out in time what to invest in to make this arbitrage. But with the internet, things changed....