Friend is getting hacked?

Hey everyone, first time posting on this sub..don’t behead me for my poor tech literacy. Apologies in advance for any typos

A few weeks ago, a friend (who’m we’ll refer to as Jake) had a Craigslist meet-up to sell a device at a local Panera. Sale didn’t pan out due to Jakes fault. During the 25 minute interaction the device that was being sold was reseted to factory settings. At the time of the interaction he did NOT have his Bluetooth on or was connected through in house WiFi, although he doesn’t remember if he had his WiFi on. After a series of events within his device he believes he’s being targeted by the once potential buyer. Today he’s received a call from the buyers company on a new (in app) phone number (like the Text-Free Apps etc) (Note: the buyer was wearing company apparel, thus tracking the number lead to the company) Upon attempting to call back, Jake found out that the company is indeed connected to the number although it is an unregistered number.

Questions: 1. Could the number possibly be “spoofed”? (Again, upon returning the call it was sent to an automated messenger stating “The number you have reached is not in service” etc. 2. Is it possible to compromise delicate information (Photos, Text Messages, Passwords, Emails etc) without ever connecting over WiFi or even attempting a secure connection? 3. Is there anyway he can genuinely find out that someone other than himself is accessing his iPhone?

I would greatly appreciate any and all responses.