Reformatting HD on G5 tower no DVD drive

I have the old G5 tower desktop with OS 10.4 on it and am giving it to a friend. I wanted to reformat the hard drive first so she'd be starting from scratch, but the internal DVD drive doesn't work anymore. I have an external DVD drive and have tried to install the OS from the original disc with that, however it is not working. When I double click the install button and hit restart, I have tried many key combos to start from the DVD, however it is not working. I have tried holding down the C key, the D key, and even the option key to select a drive, but the only one that shows up is my internal hard drive. Any other thoughts on how I can get this hard drive reformatted? TIA