whether or not it's worth it depends on why she's selling it. is she having problems with it, or did she simply treat herself to a new machine? does it come with all the presser feet and some bobbins? the manual?
grab your scrap bag (fabrics and batting so you can test it on a sample quilt sandwich) and test it for at least a half hour. test every stitch. if you won't have to take it to a repair shop before you use it, i'd say it's worth it. if nothing else you (1) have a backup machine; and (2) have more time to save up for the machine you really want.