No idea about Silja points in specific but I would be very surprised if AY rewards offered good value when booked with 3rd party points.
It's almost 100% sure they won't earn anything to AY Plus and I'd check the value carefully..most likely it's better to use points with direct Silja rewards for someone who uses cruises in the first place.
Short flights are sooo cheap today that rewards in general (with taxes..) make very little sense anyway...only at the last minute they might be good value..