As Zuvio said, Vader's ability is interesting because using it creates a new opportunity to use it.
Trigger: "After you perform an action..."
Effect: "Spend 1 force to perform an action"
So you perform an action either during your Perform Action step or from some effect like Afterburners. Vader is triggered. You may spend a force to perform an action. Vader is triggered again. You may spend another force to perform another action. Vader is triggered again.
This continues until you have no other available actions (as each action can only be performed once per ship per round), or you run out of force, or you decide to not use the ability again. Typically Vader will spend 1 or 2 force this way; it's usually not advantageous to spend it all because force is so powerful for dice modification and it recharges slowly. However, if Vader is sitting on several crits that require actions to repair, or he really needs to dodge an arc, lock, and focus, then it can be worthwhile.
It's understandable why your friend reached this conclusion -- in other games perhaps this would be usable only once. But in X-Wing, you can use an effect once per opportunity, as long as you can pay its cost and resolve all of its effects. Some effects trigger repeatedly but can only be used once because they spend a charge. Others only trigger once during a sequence of events (e.g. Norra Wexley can only add one evade per time she defends, but she can use her ability each time she defends e.g. if many ships attack her).
And I can assure you, this is how the ability works -- I've played many games with and against Vader, including in tournaments, and this is how the card is expected to work by everyone. It's also very normal-feeling once you get a feel for how X-Wing's rules work.
We're happy answering more questions like this, so don't hesitate to ask!