While it's illegal in some countries to produce such a ticket to an authority, I'm not sure it would be illegal to run the website (that's somewhat up for debate/interpretation). Is it ethical? Not in the slightest.
He has every right to ask for a developer. But I'd recommend any developer who takes this project on to consider their own liability in this project.
Also, remember, most modern airlines/security services do have the ability to verify such tickets. So it's not exactly full-proof.
There was a similar case in Canada where the operator of a similar service (in person) was charged w/ Fraud. Although, it's a tad different since his customers did not know they were purchasing fake tickets: https://globalnews.ca/news/2446148/toronto-man-charged-after-fake-airline-ticket-scam-netted-275000-police/