You should just be sending hi-res PDFs to the printers RIP. As long as the artwork is the correct resolution and size you can use whatever software you want. I wouldn't necessarily use Photoshop to originate your artwork, especially for large format.
Not sure what ONYX software is? Sounds like it's the software that runs the printers RIP?
To be honest if she's starting a printing business she shouldn't really be asking this question.