I am new to this forum(today) so I'm probably not someone to listen to all that closely. I would say that you know the answer to this yourself the best. If you find it acceptable for them to pursue this sexual relationship, then go ahead. I think she is a bit too soon out of the relationship and without knowing specific facts or anything I'd say that just open, honest, companionship would probably be better. If she still desires the sexual relationship down the line then I don't think it would be totally out of line to allow it as long as you are all willing and open about it.
Just my two cents I thought I'd add in.