Mach2 can be fussy with computer hardware. Did he try the special driver? There is a file called specialdriver.bat in the program folder. If this doesn't solve it, try a different version of Mach2. Version 6.11 is the current stable release, but there is a version 4.xx available too. Let us know how you get on.
Just a thought - there are settings for direction pre-change and for pulse width in the motor tuning sections, might be worth a look?