Probably 0-based? I'd assume they'd come to the same reasoning as we did, that indexing into an offset starting at zero works better for addressing memory.
We use 0-based, but we also invented the concept of 0. It's true that multiple civilizations independently invented it, but perhaps an alien civilization never invented it and got along fine without it.
Would they even have linear memory? That's a pretty big assumption. They might also have a completely different paradigm that we would never ever think of.