Freelancer.com may be a good choice.
BHW of course can be a good idea too. You can become Junior VIP and post in the Marketplace, start it up as a proper business, that would be a great idea i think. It costs some money, but if you can afford it might be worth the investment.
If your mum is good at writing I'm sure it'll work out well enough, with you managing the thread, and orders. Or if you get a website, you can even have it all automated pretty much so the orders go straight to your mum. You just have to do your research (or she has to do hers) and wherever you are advertising, find a price point that is competitive and/or find something you can offer that other people aren't. This will give your mum a competitive edge.
I have worked on freelancer for larger style projects and it's a decent platform.
I definitely wouldn't knock fiverr etc either. Just because it's called fiveer doesn't mean everything is $5. You just need to be creative with what you are offering, fiver has gigs, you need to find add ons for the gig that seem like they are too good to pass up on, but increase the price point to something that's decent.
It's also worth setting some money aside $30 or so it doesn't have to be huge, to get a nice flyer graphic done (there are people that make them in the market place on BHW for instance) and spending the rest on the first few months hosting. Investing in a small hosting plan ($10 per month) - will get you a basic WordPress website, at a reputable shared hosting platform. I'm sure you can help her with the site design etc. She can get e-mail then too, that looks decent. Build some social profiles, get out there etc.
If you have no idea at all how to put a website up (not sure ff you do so no offence intended, i just assume most people on BHW have a basic grasp of this) - but if you don't, I'll be happy to get a very basic one up to help you for free. You'd need to sort the hosting and domain name though, If you want $10 per month pay monthly, I'd reccomend hostgator (although their support is like a lottery) or dreamhost. -- for hostgator just google coupon hostgator, there are tonnes which give you the first month for 0.99c.
Most of all, just start. I tried to give good general advice above, but perhaps the most important piece of advice is spend a small amount of time on the essential planning / setting up ; then just start. The only sure way your mum will not make anything in writing, is if she doesn't start, so the sooner she does the better.
Kudos also as
@VOW Technology mentioned, that you are taking this initiative. Way to go!