Obviously everyone is hinting that she may be interested in a MMF threesome. I'd suggest asking her straight out. Say that you have thought about her request and don't have any objection to roleplaying but you have no interest in taking it further (and be very clear about where the roleplaying will stop for you). I'd also encourage you to have this conversation in a non-confrontational manner in order to keep the lines of communication open - just because you may not be interested in this activity specifically may not preclude you from being interested in other, ah... 'non-traditional activities'and you don't want to shut her down about kinks in general (unless you do...).
