Wrong to steal my coworker's crush?

Ok, there's this girl I work with that just recently became single, and I'm interested in asking her out. Problem is, I work side by side with a guy who is OBSESSED with her and says he doesn't want any other girl but her. It gets more complicated... his best friend is dating this girls older sister, and they're all in the same social circle basically. This guy is really cocky on the outside, but has admitted to me that he's secretly TERRIFIED of women. Well in the past she was single for a whole TWO WEEKS and he couldn't man up and ask her out. Now she's single again and he's being a wuss... AGAIN. Also, this girl has shown signs of liking me for a long time. My question is...

Would it be wrong to ask this girl out considering I know how much my coworker likes her (me and him also get along really well)?

I mean I held back and didn't go after her the first time, and even encouraged him to go for it... only to watch him totally puss out.

He told me he was gonna ask her out yesterday at work, like THAT DAY... so if he chickened out I wanna just go for it.