Is there ever a GOOD time for a Mac to go down?
Anyway, first try unplugging the whole thing, wait 20 minutes or so, then plug it all back in. This may resolve the issue.
If not, try starting up the machine and holding down the S key to bring up single-user mode. If you get video, ah good (see below). If you don't, oh dear.
If you get video on single-user mode, just type "exit" or "reboot" to get the machine to start up. We've now determined that there's no video hardware failure, which is a Good Thing. From here you should probably try starting up from the original system DVD and run Repair Disc and Repair Permissions. Disc Warrior, if she has it, would be another very good idea.
If you didn't get video on single-user mode, the assumption has to be that there's a serious hardware problem. Before freaking out, you can do the following:
1. Connect the nonfunctional iMac to another Mac computer via a Firewire cord. Start up the iMac and hold down the T key to get it to boot in Firewire mode (you might get video on this, you might not). The hard drive of the iMac will then show up on the connected Mac, and you can copy her user folder (or the really important stuff therein) onto another drive so as to have a backup.
2. Once that's done, take it to a local authorised service centre.