Just to add, though I don’t think this applies to OP’s friends’ situation, if you want to book for an as-yet unborn baby, you just book for the adults as normal and then call up BA after the baby is born and they will add them to the booking.

My son’s first ever flight was a 12.5 hour BA flight at the age of 3 months, I always found BA to be very good with infants in terms of on-board experience (long haul) and baggage allowance.