I'm in a unique position to answer this as I have a colostomy myself (2 in fact), have hooked up with girls with colostomies and briefly dated one as well through a Crohns youth support group.
For me the colostomy has never been an issue in sex. As long as the girl isn't grossed out, it plays no part or has any effect on the sex or intimacy. Likewise, I had no issue with the girls who had one - was far more attracted to their personality and faces/tits/bums etc.
As such when I saw this post I wanted to say sure go for it, wouldn't bother me.
However, as much as I feel hypocritical saying it I very very much doubt I would see a WG with a colostomy or pouch of any kind.
When we punt we are buying the illusion of the "perfect" girl. And ultimately stomas are a disfigurement and are not sexy. Whilst the stoma still doesn't bother me per se, I can't imagine I would ever pick to see the girl over any normal SP without one and I imagine this might be the same for a lot of others. There is a difference between dating, relationships and civvies who come with good and bad, warts and all; and punting where you only want the best, even if it's just for the hour.
Dani raised a good point about using it as a USP to those with a medical fetish but that would actually worry me. You should never EVER penetrate the colostomy itself and whilst the thought will make most of us shiver, I know this is something that does happen and I would fear that a sick individual would take advantage and cause permanent damage to this girl.
So all in all, if she wants to do it go for it but she should be fully aware of the risks and that it will put off basically everyone without a medical fetish. I am in the most understanding position possible when it comes to all things stoma and even I am realistic that I just don't see myself going with someone with a pouch.