You could simply ask him to show you the Bitcoin purchase TXs, and visualize the current balance as well as whether there are output TXs and so forth. I suspect though that you currently don’t really understand all this, and have to take his word for it. You could nevertheless get acquainted with this all to develop a minimal understanding, and ask to see what I commented at a later stage.

A lot will have to do with the deal you made with your friend. If you lent him money to invest on your behalf, then you should have set some conditions (i.e. supervision, timeframes). Price could go up or down by December, so you really need to be prepared to any of these scenarios. If by any chance he is performing day trading, the same could happen, and it’s frequent for people to end-up losing in the event, not winning.

Nevertheless, you could write down the price of bitcoin when he roughly made the initial purchase, do the same at selling time, and calculate yourself the resulting approximate benefit or loss. Any figure that he presents that deviates substantially from this calculus is sus …