"In the back" they can be Qualified crew (Supranumeraries) acting mainly as client "Representatives", mainly due to language issues to permit better communication with the Pax. They can also efectively be Cabin Crew on the flight, but before that, HiFly would have to provide this crew some training, and then the HiFly local authority would allow them to act as crew. Not much training would be needed as they are experienced on a similar type (A330), but still, believe some training would have to be provided before they would be allow to act as Cabin Crew. Done mainly in the case of slight longer contracts (1 month or more). Saves quite some money to the client, on this case Avianca.
"In the front", believe that without an EASA "ticket" there is no way around it.
Sorry for the thread creep ...