However, women's shoes are often far more narrow than men's, so you may want to err on the bigger side, go for styles that aren't so width dependent (e.g. no pointy toes) and look into brands/styles that come in wider widths. If you're ordering online, asking for the width of the widest part and height of any potential problem areas wouldn't be a bad idea either.
I wear a women's size 10. If I buy men's shoes, it's a size 8. So, I'd go with an 11, or maybe a 12, since you're looking for heels. I know I always go up at least half a size when I buy heels, just to allow for a little more comfort.
It probably depends on the brand you want to get. From just the different shoes i buy i can range almost a full size and a half. So if you can bring the guy with you, or just measure his feet or something like that.