If a woman exposes her cleavage to you, is there anyway to let her know that you liked it without coming across as a total perv?

My next door neighbor is pretty hot. Today she was wearing a shirt with a plunging neckline. She looked really good. She and I were talking and her kid was at her feet. A couple of times she had to bend down to help the kid with something and the view was amazing. Nice, big C or D cups in a cute little silver bra. The plunge was so great that I could see her belly button, if you know what I mean. The shirt fell freely away and I could see her whole chest.

So, is there any way for me to thank her for the show without being a perv? I'm happily married and would never cheat. She and my wife are pretty good friends. I know that her marriage is on the rocks and that she and her husband only get it on about once every couple of months. She told my wife that the last time she tried to come on to her man he couldn't get it up. Poor fat cuck. I would not touch her. But I'd love to keep getting a show when I see her. How do I say thank you?