nvu was the first thing that jumped to my mind, but I haven't touched it in a couple of years at least. I suggested it to someone in a similar situation. It looks like it hasn't been updated since 2005.
MS FrontPage was designed for this purpose and may be on her machine already with certain versions of MS Office. Otherwise it'll cost. I haven't used it since the early days and hated it then, but it's been quite a few years and presumably there are improvements. Hmmm, it looks like the FrontPage brand has been discontinued with similar function being picked up by the SharePoint tools or MS Expression.
(descendant of Netscape suite and Mozilla suite) has a composer that should be easy to use. Again, haven't used it since Netscape 4.7 I think.
is a browser/editor combo that's been around a long time and apparently is up-to-date. I haven't used it in a few years, but it was a bit awkward for me and may not be the thing unless they've changed a lot about it.
I'd be a bit surprised if there aren't several free online editors (besides those tied exclusively to their host sites), but I haven't checked into them and wonder if they'd be easy to manage the files with.
EDIT: Even though nvu is a couple of years old I'd probably start with it or SeaMonkey given my past experience, the "dummy" moniker and their cost (free).