Sylvia is one of Grace's social acquaintances and a fellow parent of a Rearden student. Like Grace, Sylvia herself is a Rearden graduate, but the two weren't incredibly close when they attended school together. She's an employment law attorney and a single mom to Daisy, a brilliant 7th grader whom Sylvia adopted from China.
The rest of the ladies on the planning committee are essentially interchangeable (besides Malaga), but Sylvia makes an impression for two main reasons. First, she's the only person from the NYC social circle who helps Grace after the news about Jonathan breaks. She hails Grace a cab as she runs out of Henry's school, then snaps into Olivia Pope mode and tells Grace what her next steps should be to avoid the media feeding frenzy. Second, she reveals that Jonathan came to her for legal advice after being fired. She told him it wasn't a good idea to sue his former employer, and he rudely stormed out after hurling some profanities at her.
These instances are significant because they reveal how little Grace truly knows about everyone in her life. Even Sylvia, a minor character in this story, gets a better glimpse at Jonathan's true colors well before his own wife does. Considering that Sylvia knew he had been fired months beforehand, we wonder how she managed not to even raise an eyebrow any time Jonathan's oh-so-demanding hospital shifts were mentioned in her presence.
Also, Grace's shock that Sylvia has crisis management experience borders on the edge of unbelievability. Sure, Grace is processing a lot when Sylvia rescues her outside of Rearden, but "helping clients navigate sticky situations" is pretty much the job description of an employment attorney. The fact that Grace has never considered what her friend's professional life might entail just goes to show how content she was in her isolated bubble.