I am confused - when you say all in one you do not mean a printer with a fax built in then? That would be my first assumption from the phrase "all in one" but in this case would make a mockery of the question.
So do you mean it is a printer with built in scanner? If you can scan a document I believe there is software that will enable facsimile transmitions to be "emulated" and maybe the best first stop would be to see what was shipped with the printer or visit the Epson web site. Maybe others out there know something about Epsons - myself I have only used HP gear, and the all in one PSC unit I had could fax in it's own right. But I am sure I have seen software to address this.
Confirm I have got question right and maybe someone will recommend some for you.
Have her check the software package that came with the device.
With my Brother scanner/printer all you have to do is click the device in Explorer, click PC Fax on the pop up menu and then click send. It will scan the item and send it over the PC's modem after prompting you for a phone number.
Option 2 is to use the winodws fax service like Zombie said.
I've tried a bunch of different fax programs over the years and they all had an offensively steep learning curve.