If a crook steals my phone number, what is there to stop them from stealing my bitcoin?

Like Robert mentioned, paper or offline wallets would have protected your friend in this circumstance.

It seems like recently a lot of online bitcoin and cryptocurrency wallets are being hacked or attacked and coins are being stolen so it is probably your best bet to go offline if you want to stay on the safe side.

I recently was made aware of actual physical wallets which are probably even better. Here is one: Ledger Nano S

I don’t really know what to say about the Verizon trickery thing. If that is possible, maybe if your friend was monitoring his Gmail account then he could have prevented the gmail takeover (they probably sent a warning email). I’m sure there is some solution to this type of scenario but i’m not aware of it.