Your friend doesn't tell you everything. It's technically difficult to hack a wallet because of mnemonic problem.
It's on the user side I guess why that unusual activity happened to your friend.
And next time, try to use another reputable wallet and not the mentioned one by you as I didn't aware of it.
I have the same suspicions as well, it is easy to think that you are doing a good job and keeping your computer clean because you do not see anything wrong going on with your system, but if you really begin to try to find malware in your computer you will see that most computers today are infested with malware, and while some of that malware is not that dangerous there are other pieces of malware which are waiting to send out any important information they can collect out of you, and as such I think this is the main reason why the friend of OP lost his coins.
Another reason might be, OP's friend is a victim of phishing.
Likely ended up for let's say airdrop and asking to connect a wallet. But in the process, there's a prompt to enter the passphrase or mnemonics which should not be a step on connecting wallets as that should mostly do with the legit browser extension.
Anyways, OP didn't come back for another information that might help with the case.