Unless, (hopefully) you get very lucky and find someone here that has the exact BIOS you need, your friend should seriously consider the overlay software. I know that is not the best way to go but I just did some intense searching via Google and all point to one of two pages; neither of which is any place to get the BIOS files or an AETON site. It's like they just don't exist. The only other slim possibility is if that board is a "clone" of some other manufacturer and you can use their BIOS file to update it. But I'm not sure if or how you can confirm that possibility.
On the bright side (if there is such a thing in this case), you can use the overlay software for now and if you do find a BIOS in the future, it is possible to remove the overlay software without causing harm. At least he could use the HDD now if you do that.
Good Luck!
PS. One other thing you could do is go to someplace like Drivers.com and put in a request on their board. I did find a modem driver for AETON there but no BIOS. It might be worth a shot doing the request. Maybe one of their members can dig one up for you.