Ask him to do this:
1. Click on his Bitcoin Core wallet, the three dots at the top-right corner and "Show my recovery phrase". This should show his seed.
2. Download Electrum (electrum.org), create a new wallet by clicking on "I already have a seed", write the seed from the app, click Options and select "BIP39 seed", click next and select "legacy (p2pkh)". He should then have access to his wallet.
He's not running the Bitcoin Core wallet. He's running the Bitcoin wallet by Bitcoin.com.
I know. I mean, in the Bitcoin wallet by Bitcoin.com, they call Bitcoin "Bitcoin Core". I'm talking about the iOS wallet. I see how this looks confusing.
When you open the app in the new version (redesigned), there should be a card "Bitcoin Core", where he should click to get the seed and do the steps above.