It seems pretty clear to me:
Preboarding is available for Customers who have a specific seating need to accommodate their disability and/or need assistance in boarding the aircraft or stowing an assistive device.
The conjunction is “and/or” which means at least ONE (but not necessarily both) of the two conditions must be present in order to preboard.
Needing a wheelchair down the jetway implies needing assistance in boarding and stowing the wheelchair. This qualifies your friend for preboard, regardless of whether or not they need a specific seat.