For sure a concern, but still seems like a reasonable action given that those guys didn't look like they wanted to sell Tupperware.

It's a definite possibility and so is the resulting liability. However, unless that concrete is a facade or otherwise very thin it will probably stop handgun rounds up to even the .45 caliber size. If he's using defensive rounds (hollow points), then penetration through concrete is reduced even further as they are made to expand and dump their kinetic energy within a very short distance after the expansion is triggered. The bullet damage to the wall is something he will certainly be liable for though, also his new screen door lol.

See the following for an example: https://www.youtube.com/watch?v=qFwntHIWjaw

I'm wondering if this is why they shot out their door at an angle instead of straight out. Not very likely for the bullets to go through concrete, but the door across the way? Yeah, a definite possibility. If the guy had that much presence of mind in that situation then good on him.