That's a good point about added risk, but it may make him more secure depending on what kind of threat he is trying to protect against. If he is worried about physical attacks on his person, then deleting the app might be safer. If someone was to forcibly take his phone and find a bitcoin wallet app, even one with no coins, they might be inclined to use physical force to get him to reveal the location of his seed phrase. Having no wallet app present may prevent this.

If you are very concerned regarding theft or hacking, then your best bet would be to either take the simple option of buying a hardware wallet, or the somewhat more complicated option of setting up a wallet on an airgapped (no internet connection) PC. Both of these would be much more secure than using a mobile wallet.