A late 2009 iMac will run Sierra with no fiddling. An early 2009 will run it with a bit of hacking, and will run El Cap as-is and that should be fine -- certainly much later than Mavericks. The 2008 (and I think 2007) iMacs top out at Snow Leopard officially but I believe you can get them to El Cap with a bit of fiddling. Yes, please double check exactly what she has.