From the Wingly website FAQ page, In response to the "Is it legal?" section...
Yes. According to the European regulation (see art. 6 paragraph 4a regulation (EU) no. 965/ 2012) pilots are allowed to share flights as long as the aircraft does not exceed 6 seats and has a non-complex power unit. Furthermore pilots are only allowed to share the costs, they are not allowed to make any profit on the flight. Wingly's policy is that the pilot pays the same share of the flight as the passengers, or more. We have received written confirmation from EASA, stating that flight sharing and the advertising flight shares is totally legal, so long as the costs are equally shared. We also have the same letter of confirmation from the Civil Aviation Authority (CAA) in the UK. In other words, because the flight remains a private one, the pilot does not need a commercial pilot’s licence to share his cost with passengers.
Forgive me if I've been misinformed for a long time, but isn't a Mooney classed as a complex, on which this flight occurred?