There are two areas in the 9th Arrondissment that I know of as places to avoid...
The area between Boulevard de Clichy and Rue de Douai. These two streets run parallel at the northern end of the 9th Arrondissment and they constitute part of the 'red light district' (although prostitution is illegal in Paris). There are lots of triple-X shops and clubs that attract the adventurous and lure them in to scams. There are lots of transvestites roaming about along with straight prostitutes and this crowd attracts drug dealers and pickpockets. I wouldn't even think about entering a club or restaurant in that area. While I have taken tourists to the shows on Boulevard de Clichy and then night walks up to Montmartre, these areas are safer (large groups also make a difference).
The other area is on the eastern edge of the 9th Arrondissment in the vicinity of Gare du Nord. Although the area is heavily patrolled by the police, it's still a no-go zone and likely to be especially dangerous at night. I once saw a person get murdered there (a victim of an aborted robbery attempt), it was in the station itself so not in the 9th, but the whole area is volatile. It's also a tinderbox for racial tensions.
On the other hand, the south side of the 9th is relatively peaceful and relatively safe. It's generally safe from the Opera all the way up to the Lorette church.
You mentioned your friend's appearance is 'stereotypically lesbian'. This may be a mitigating factor in the area around Pigalle, but not in the area near Gare du Nord.