yeah calling her a cheap sh*g is a little harsh but he isnt the only one to blame here,
if you are fully aware that he has a g/f then there are 2 ppl at fault here,
i have ( like a lot of people) been cheated on, and the pain is worse then anything i have ever experienced in my life! you could have stabbed me 1000 times in the heart and it wouldnt have hurt half as much as when i found out!!
so i see it all from the g/f point of view, its not even as if he has fallen for you! no that that makes it any better but at least her getting hurt would be for something,
where as a bit of fun once a month is nothing, if that is all it is why not go find someone else ( g/f less) to sleep with