Sky, the answer to your question is... "it depends".
One-night stands don't fall under the poly umbrella technically speaking. It implies a random hook-up with no emotions involved, where emotional connection is one of the key components that define the polyamory concept.
My impression of the intent of your agreement is for one partner to have some protection if the other is about to bring extra drama into the relationship dynamic by sticking dick in crazy or vice versa.
Your problem is that this really isn't a one-night stand.
She's been talking to him, but you haven't really indicated if she has an emotional connection with him, or if this is purely a physical encounter. If its just sex, then I would say this situation isn't covered by your agreement. Like SourGirl says, (if I'm reading her right) if it's just sex, its a non issue because he wont ever be part of the picture.
Now, if she does have an emotional connection or there is a substantial possibility that an emotional connection could develop, then I would say your agreement does apply in this situation.
In which case, your problem isn't him. Its her.
You are going to have to decide, if she's willing to throw agreements out the window whenever NRE strikes, whether or not the benefit you receive from relationship is worth the additional grief she generates.
Me: 48 - Married, straight, male
Shiela: My wife.
Suzanne: My FWB
Adam: Shiela's LDR