i would first suggest you use $0 for cost and then use cost equal to the sales price. what's the difference in taxes? could be $0 since LTCG gets preferential federal tax treatment. also, look at the state returns. once you have an idea of what you are looking at in additional taxes if $0 is used you'll have some idea of what to do.
if there's a date acquired provided perhaps you can go on the web to do some research. in the end it's your girlfriend's return and her decision to make.
