It's "proverbial".

Act as though you heard nothing. They have enough drama in their lives without worrying about the neighbors!

Do nothing because its none of your business unless you are apart of the love triangle as well??

Do nothing! Its absoloutley none of your business, and you REALLY should not have been listening in the first place.
"but couldn't stop myself from listening." Is not an excuse.. You shouldnt have been listening. Period.

Well its not like you went outside with the intention of listening in on an argument that you didn't know would happen! However, what's done is done so just be polite, don't mention it again and go on with your life.

We sometimes hear things we shouldn't but the best course of action in those cases (where you're not involved) is to simply ignore it!

I agree with the ones above, you should not bring it up again, just act like nothing happened, im sure she is feeling bad enough she shouldnt hvae to worry about the neighbors talking about her.
~*Snooky*~

I don't think you're necessarily wrong for listening. If they're going to be loud about it, then they have to expect that they're going to be heard. What's important is what you do with that information. If you gossip about it, then that's wrong. If you keep it to yourself, then everything should be fine. Was there violence involved? If not, then just keep what you heard to yourself.

Deny deny deny

It is none of your business so to acknowledge it would just be rude. There are always going to be times you hear your neighbors it is up to you to do the right thing. If you were outside put out your ciggie and go in.

I agree - do nothing. When you see her act as you normally do. Don't say or do anything that you don't normally do. She feels bad enough already without you treating her any differently or making her feel uncomfortable. Unless she brings it up with you just forget about it.

I think just act how you normally would towards her, exchange pleasantries and be done with it. Don't think bringing it up in ANY way will smooth things over, it'll just become even more awkward.

You shouldn't do any avoiding, just say hi and be cheerful, she probably needs to feel like the world around her is normal since her personal life is going through some hard times. If she wants to talk to you she will reach out. You should keep the info to yourself and not talk about it to anybody else, especially people that live in the same complex (if it's an apt).
Also, don't go above and beyond what you normally do, she will tell you are up to something abnormal.

If she knows you heard everything, just come right out and ask her if everything is ok. She may welcome an unbiased ear to listen to her.

Act normal. It was none of your business so treat it just like that. Those who chose to argue within the vicinity of others are the ones who should be emabarrassed. If she ever brings it up, I wouldnt lie to her. Acknowledge that you overheard somethings and just hope she is ok.

I've heard my neighbors arguing and I know they've heard me arguing...it's part of living closely with other people. The polite thing is to not acknowledge it, at all. I absolutely can understand not being able to help overhearing, and whoever thinks that it's possible to turn off their ears is being silly--when people are screaming you either can leave or listen, and leaving isn't always an option (or even something you'd want to do).