For your board to talk to the device, you will need to stop using the FTDI converter. you have interface incompatibilities here.

As I understand it, this is you setup:
PIC ->non-inverted serial->FTDI->usb->PC (with virtual com driver prolly)
Other device->true serial->USB adaptor->PC (with virtual com driver prolly)

Just connecting the USB to USB will not work.
connecting non-inverted serial <->true serial will not work

I see 2 choices:

Add a level converter (MAX232) to the PIC serial then connect that to the other device true serial
or
Use 2 new pins and SERIN/SEROUT so you can make true serial data without the converter. I think this should work unless the other device needs RS232 levels (-12 to 12 V). In that case you will have to use a MAX or simular converter.