Doug,

This was a tough enough question that it forced me to figure out my BBS password.

I wonder if FRS radio might work over such a short distance, concrete or not. An example kit:

http://www.newegg.com/Product/Product.aspx?Item=N82E16872104053

So, essentially fancy walkie-talkies, push to talk. Something you could configure before use.

Good on you regarding your visitor.

Jim