It was no big deal, since when we went home we were still in the tag-off window, so she could just tag-off and instantly re tag-on.
CalTrain has been putting in some effort to reminding people to tag off, but the results of this effort are pretty underwhelming.
Menlo Park is a nice "barrier-free" station where you can just get off (at least from the SB platform) and start walking to where you want to go. This is a good thing in terms of usability, but means there's no specific exit locations to put up tag-off reminder signs.
So here's an idea: How about stenciling reminders to the platform? And to make the reminders more useful, include an error to the nearest Clipper Card reader? Like this:
(I'm drawing this from the point of view of someone getting off the train, but as you can see it's designed to be double-sided and work for people about to get on, too).