In my mind, a role play scene between both women would focus on a type of "before" and "after" construction of Ali. They can each give the other the missing pieces about Ali's identity.
Ali's English girlfriend is briefly mentioned in the story's opening. She can fill out the picture of Ali before his hard turn toward a fundamentalist brand of Islam. Kurieshi himself gives some glimpses of this in the details of how his room was once littered with "the usual tangle of clothes, books, cricket bats, video games." These are the details that the girlfriend can confirm to Bettina. She can provide insight into how Ali was as a student in terms of how he pursued his studies and his extracurricular sports of football, cricket, and swimming. She can describe the pictures on the wall of his room before they were taken down, as she had visited the house enough for Parvez to be conscious of her absence.
Bettina should be asking two fundamental questions to the English girlfriend: what happened, and how could you be with someone like him? Both questions can be answered in the girlfriend's construction of what Ali used to be like. She can give insight into Ali that Bettina would not have been able to envision. Given her last encounter with Ali in the cab and running from it into the British night, there is a very good chance that a conversation like this would be quite illuminating for Bettina.
It makes sense that the girlfriend has little idea of the massive extent to which Ali had changed. He would not have kept in touch with her. Her fundamental question to Bettina is probably something like, "How bad has he gotten," or more directly, "What's he like now?" This is where Bettina can expand. She can fill in the picture that the girlfriend is missing, just as she filled in what Bettina could not know. Bettina could talk about the beard he has grown, the fact that Parvez and she picked him up in a poor part of town outside of a mosque, and that his entire demeanor has changed. Bettina would have to talk about how he disrespected her in the cab in terms of the things he said and the attitude he displayed.
There could be a shared sense of sadness as to how the choices that Ali has made entirely changed him. However, I tend to think that both of them would be relatively happy that they no longer have to deal with someone who is as intense and narrow in their view as Ali is. I think that Bettina's picture could make the girlfriend kind of happy they are broken up. This might be where the role play exchange could end. Ali's fundamentalist position tends to marginalize Western women. This can be a point where Bettina and the girlfriend find a sense of community in the exchange.