I am assuming that as programmer, you have completed the unit/system testing so your partner should focus on the acceptance testing such as usability. I have found creating personnas and putting yourself into the position of those personnas a good starting point.

I am assuming that you have done the unit/system testing as the programmer, so your partner should look at the acceptance testing. I have found creating personnas (of your end customers) and testing as though you were one of them a really good starting point.