Hey! I was going to ask what an IEC visa was, then I googled it and it turned out to be exactly what I've been in Canada on! (also known as a working holiday visa)
Having been here on a working holiday visa for a year now, I can answer your question. In short, whether travelling with this visa or as a visitor, your girlfriend will always need proof of a return flight. It's one of the conditions of her working holiday visa through BUNAC and it's a given if she's entering as a visitor. If she presents as a visitor at the border and can't show proof of return flight, they have every right to send her home. Heck, last time I travelled to Canada from England (mid-way through the duration of my one year visa), I even had my return flight confirmation checked by an immigration official in the check-in line at Birmingham Airport! This was even before I'd checked my bags or got anywhere near security!!
She could enter Canada early as a visitor, but she'd need that proof of return flight. HOWEVER, she'll receive a letter from the Canadian embassy which she'll need to present at Canadian immigration in order to get her visa issued. This letter will be sent to her UK address. Also, although she'll be in Canada earlier, she cannot work without her visa and will have to land again at a border crossing once she does have that letter.
Personally, I'd try and delay that flight for a few weeks. Also, if she needs a return flight that she won't be using, there's 2 things I've learnt:
1) Air Canada return flights can be changed for a GBP50 processing fee (plus costs of any difference in fare) - at least this was the case when I booked my flights through STA Travel in the UK. So I change my return date so I could go visit home half way through my visa
2) Canadian Affair charges GBP100 for a flight that is cancelled well before departure (check their website for exact days - I think it's 70) That was the cheapest option for me when I needed proof of return flight but was never going to use it (I'm now applying to Permanent Residency and am living in Canada while it's processing)
Hope that helps! Good luck!!