Altaria (when outside their territories, anyway) tend to be very benevolent, actually. If you see it again, it will likely not expect anything at all, and it might not fully comprehend gestures of gratitude.
On the other hand, if you look out for any swablu hatchlings or fledglings you come across, that may be enough. For example, if you see an injured or sick swablu, consider giving it a hand. If you see a predator attacking a swablu and you have the pokémon to defend it, consider doing so. Or simply construct a feeder or shelter fit for swablu in your hard if you have enough space. Even the little things can go a long way, but the important thing is to repay kindness with kindness.
But resist the urge to feed or give toys to the altaria who saved you. This might cause it to expect you to feed it regularly, and unless you want an altaria on your team, that may be a bit of a problem.
Best of luck!