Really, Singer makes the best machine for your money. Brother machines are pretty good as well. So far, I have seen many people suggest going to Wal-Mart and getting a $100 machine. These are good machines and will get the job done. The little 20-40 dollar machines from hobby stores are useless if she wants to make anything to use. They don't do secure stitches, and are only really good for scrapbooking or decorating things.
Honestly though, what I would suggest is checking resale shops and yard sales, and finding an older machine, from the 60's or 70's. There are 3 reasons for this.
1. They are more durable because they have more metal parts. They will last longer if treated properly. (I use a Singer 500A, made in 1960, the last machine made with FULLY metal parts, which they claim will work for 100 years - so far so good.)
2. You can probably get one for around 20-50 dollars (Really - I paid 20 for my Singer, which is built into a table).
3. (And this is the most important reason to me) The machines made back then have a reverse stitch lever, rather than a button. With years of experience sewing, I still detest having to hold down a button to sew in reverse. A starter should have both hands free to guide the fabric and learn the ropes and all. This will make it easier.
I hope this helps!