Yes as you say it's reminding her that she's required to declare online, paper declarations aren't acceptable.
As long as she has her espace set up on netentreprise she should be able to declare when the time comes, but if you declare quarterly there are specific windows when you can submit your declaration. Usually you get an email to tell you when the window has opened. For Q1 you make your declaration during April. You can't make a quarterly declaration in the middle of March. I imagine that's why you can't access the declaration at the moment but usually when you log on at declaration time, it is offered to you as soon as you log on to your espace.
However from the letter it looks as if they are ticking her off for not having done anything about it yet so maybe she makes her declarations monthly? In which case I don't know how it works what the windows are. Did the previous letter ask her to do something specific? Has she set her account up?
Edit - re reading your post I'm wondering if you're trying to find your way round the site without logging on to your espace. You can't; once you log on, you get a completely different set of menus, relevant to you. You can look at your previous declarations, see totals for each year, download various attestations, check that your account isn't in debit, send emails with queries, etc, and also, submit your declaration when due.