A question about a roof issue in my pre-purchase home inspection

Howdy all! I am under contract to purchase a house, and got my inspection report back last night. One of the only real issues that the inspector found was on the flat roof over the master bedroom. He found that there was about 1.5" of standing water pooling on the roof. We asked the owner about it, and they said they had Hydro Stop applied so standing water shouldn't be an issue. Based on the research I've done since then, it seems like Hydro Stop is a pretty solid product.

I'm wondering if y'all think this is something I should make a big deal out of. I'm not sure what can be done to change the grading of the roof enough top make sure that water doesn't pool, or how expensive that would be.

Here are some pictures: http://imgur.com/a/DFTA9

Thanks!