Let's get the rules clear to start with:
If you earn
anything (other than as an employee on a PAYE basis) you're required to
(a) register as self-employed ;
(b) register the name and nature of your business(es) ; and
(c) submit an annual tax return.
Even if (as friend of mine used to do) you simply run a monthly tea dance in a village hall and (after you've paid for the hire of the hall, the fee for the person providing the music and for the refreshments) you typically make no more than a tenner profit from each dance, you MUST register your 'business' and complete a tax return. (My friend found that out the hard way, when HMRC started sending her very nasty letters, as they'd somehow found out about her 'business').
The obligation to submit a tax return applies even when there's no tax to pay (or, indeed, if your business has actually made a loss). The fact that HMRC hasn't sent out a form is irrelevant; the obligation to submit a return still applies.
However two years ago it became clear that HMRC would write off the £100 penalty for anyone who could offer a 'reasonable excuse' for not submitting a tax return on time:
http://www.independent.co.uk/news/uk/home-news/hmrc-told-staff-to-waive-late-tax-return-fines-10286495.html
Simply stating that she erroneously believed that, due to not receiving any forms, she wasn't required to submit a return might be all that's needed.
For the future though, your daughter should register to submit her returns online. It's MUCH easier than doing it on paper because the computer asks you a few initial questions and then only asks ones which are actually relevant from then on (instead of having to work through page after page of irrelevant questions on a paper form).