It actually depends on what Emma the intern had access to.
For all we know she saw IP addresses, names, email addresses, physical addresses (from competitions) for people, plus years of comments they'd made on boards. Users aren't quite so anonymous to Mumsnet if someone there grew particularly interested and had a vendetta against them.
I don't think we ever had confirmation if there's any date stamps/ways to check which person's checked out records. MNHQ had no idea what she'd taken and still don't, except what she's told them.