My gf, blue, just booked for me, silver. Once she'd added my BAEC number she could select seats etc. Of course it then appears on my app, so if you wanted to keep it a surprise it wouldn't.
With cash bookings its the same, a friend made a booking for 5 of us, I told him to enter my BAEC number, and we selected seats. I can log in and select them, and so can he.