Just to make it clear that your silver seat selection (and other benefits) would only have applied to your friend if you both had been on the same booking (so same flights and cabin) right from the start.

You booking the. a separate flight from your account does not get them your status benefits even if you paid for the ticket hence why you are being asked to pay to select their seat.

The best you can hope for is a reward seat opening up to get them into PE and then hoping that the seat next to you is empty at OLCI or there is an empty pair in the cabin.

as to self downgrading I would do this on board via the cabin crew rather than just approaching the person sat next to your friend.