My friend and I have a regular dispute about the rules of cover gained though vehicles in between.

As I understand it, if you shoot between the legs of an AT-ST as in one squad on one side and one on the other, the target would gain light cover.

He insists, that shooting would not be possible in the first place, as vehicles can block LOS and has the opinion, that shooting across the base qualifies for this.

You are right, the troops gain light cover (or heavy in the case of the occupier tank)

To block line of sight, the mini has to be completely hidden from the attacking unit.

Your friend probably confused movement block (what the AT ST does) with block of line of sight.

I had to correct someone on this at a recent RPQ tracked and wheeled vehicles provide heavy cover. But otherwise it's light. If you can see any part of the figure including the base you can shoot it