I am on the other side of the fence - lol! I have only purchased used machines. And to top that off almost all have come off of Craigs list. That being said.... I would not hesitate to purchase a used machine from a dealer if they offered a warrantee and had a good reputation. Here is my recommendation .......
I sold Vikings for several years but now all my big machines are Pfaff. I have gravitated to them because of the built in walking foot. My best advice is that for the most part - all big name brands make good machines . The driving factor for your friend should be options such as number of stitches/type, whether she want a needle up/down selection, does she need a needle that can be moved left/right .... etc. Then she should go to all the dealers around her with a price in mind and look/audition all the machines that fall into her price range! Make sure to to tell her to purchase one with a warrantee and if they are non negotiable on price - they will more than likely negotiate on accessories! So if she likes a machine and they tell her they are firm on price tell her to ask for additional feet (walking foot, welting foot ....), embroider hoops (if she goes that route) or any other accessories that look useful. If she gets a good offer from one shop but likes a different machine at another shop - tell her to say "Well so an so gave me this offer on that machine but I might like this one better - what can you do to make the deal sweeter for me"! Also make sure that they will offer her at least a few classes on how to use her machine.