If I understand correctly, the OP sent the email because he knows that this specific FA generally does a great job, because he's been on her flights before, and that she was upset, and he wanted to stick up for her in case another passenger made a service-related complaint. To me that deserves a ^. Everyone has an off day for hundreds of reasons and complaints should be tempered by praise if they're both coming from customers.

OP, I'd just leave things as they are, no need for follow-up. Despite the tons of customer service related complaints about US (try Facebook or starters), and accusations of US being unresponsive, I actually find the CSR's to be quite responsive (status helps), and good about escalating to the right teams. You might not ever hear back other than boilerplate, but I'd say chances are good that the FA's supervisor reads your email and asks the FA what's up. Whether or not the FA tells her story is another thing, and totally irrelevant to you but you've done just right as a customer.