Suggest going to a dealer to see what trade-in's they have available. A nice basic machine. Does she want to start with quilting, or does she want to start with garment making? If quilting - pretty easy as all she will really need is a straight stitch machine and not a whole lot of extra presser feet. If she wants to make garments she will a need a little more (especially in the way of feet). Also check Craigs list, but I would start with a dealer as those machines have been serviced and she'll get some (albiet limited) support. I was at my dealer yesterday in fact and they had three nice old machines for $75 - a Kenmore, a White, and something else I don't remember. They were cleaned, serviced and ready to go. If she's coming in May (and is willing to wait that long for a machine), research the dealers now and if they don't have one available "today" leave your name and number with them if anything should turn up.
If she spends less now on a machine to see if she likes it, she can spend more on other needs (fabric, thread etc), and if she likes it and wants to jump to a machine with more bells and whistles, she can keep this one as a backup (we all need a backup machine!).