Hi I'm sorry you have a friend that is crying. You haven't given me any thing that I can even begin to help you. Google search doesn't show any thing about mdmx.sys file. The F10 key takes you into the BIOS. If you don't know what your doing in the BIOS, there's no telling what harm you can do. I don't even know the operating system or the model number of your friends PC.
Here's the link for conexant modem.
http://www.conexant.com/support/Pages/default.aspx
You might try unplugging the power to the modem and see if it will reset. The best I can tell is your friend has a dial up modem and I don't have any experience with them.
You could try setting the BIOS to default. That's the best I can with the information you have provided. May be others will reply latter. Here's a link for that!
http://h10025.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&docname=bph07110