-snip- and he bought a second laptop to have multiple screens, just for crypto.
Thanks to you, your friend can now afford another laptop ;D
No seriously, there's no way to check his activity unless you're monitoring him closely.
All you can do is to get an assurance that he has the amount of bitcoins close to the borrowed money's BTC value at the date.
Ask for proof that it's in his wallet/exchange ready to be sold in December (
Note: BTC doesn't always bull in December).
For "
non-custodial wallet": you can ask for a "
signed message" with the address that hodl the fund. Then take note of the address so you can always check if he spent it using a "
watch-only wallet" or a blockexplorer.
For exchange or non-custodial wallets: visit him so you can check if he has it and ask him to move it to a non-custodial wallet so you can always monitor the funds as said above.
Resources: