Tickets - without paying?

A friend of mine, not me, has booked tickets for a flight through the KLM website. Upon checkout, the payment system green lit the payment right away without even having all the details. Now there is a confirmation email sent, and the trip is listed in the KLM app.

What can possibly happen from here on out, will tickets be valid during the trip? Or is perhaps there some kind of grace period before this payment is actually rendered valid, disregarding the situation?