These encounters are sort of bugged. Once you accept the mission and refill fuel (or repair) their ship you need to wait for them to jump out of the instance before you do.
Otherwise the mission will not complete and the instance will persist. If you enter the instance again it will seem to repeat as before but this time there is no way to complete it as the ship will never acknowledge the refuel/repair but will just sit there forever.
On the plus side they always seem to high wake so I started carrying a Wake Scanner and have been getting some encoded data, don't know if any of it is good stuff but it can all be shuttled across at the traders.
Edit: I forgot to mention that if you receive a Wing Trade credit be sure to redeem it before leaving the system as they can't be turned in anywhere else and will be stuck in your Transactions tab until you do return. Also they can be laughably low valued, I have gotten Wing Trade credits as low as 10 credits