For safeguarding that amount, he should have bought a hardware wallet. Even those have their weak points, such as firmware updates, necessity to keep a backup of the 24-word seed phrase etc. NGRAVE seems to have removed many of the risks, but their product is delayed. Time will tell.
Very recently I absent-mindedly made a similar mistake to this case. I used an encryption program while being connected to the internet. It could have led to damnation and ruin. People who own bitcoin or ethereum via exchange-listed ETNs avoid a︀ lot of headache pain¤#"!