I have never tried to personally do this, but from what I've learned just playing with the system, it knows if you are an on site guest or not (hence, allowing you to schedule at the 60-day mark). Even if your friend links her brother's ticket to their account, I would assume the window wouldn't open for him until 30 days out. I would recommend making all of their reservations at 60 days and then trying to schedule his FP+ reservations at compatible times. Even if the return windows are not exactly the same, as long as they overlap at some point, they could all ride together.