Yes I can compare. Have done many RC cruises and only 1 Seabourn but would sail Seabourn again in a heartbeat - and there's no e at the end folks π. Lucky people who get offered a Seabourn cruise. We were lucky enough to know someone who worked there so got a generous discount. Sadly he's now left and it's only the price which would prevent us from booking again.
Whilst I wouldn't know whether the people are wealthy it's not cheap to book so they won't be on the breadline. We certainly didn't find them to be snobby or standoffish. The age demographic was probably older than RC as its not a line which attracts children or party people but I wouldn't describe them as elderly- more of the age of Celebrity/Azamara cruisers.
There are no formal nights on Seabourn. People tend to dress nicely so you won't find scruffy shorts and baseball caps in the restaurants but suits are not expected. There is one special night - called Gala Night - where people did dress up a bit more but nothing like the old 'formal' night on RC.
Entertainment is obviously different as
They don't have the size of ship for big production shows. The shows take place in a cabaret lounge setting with a small cast of high quality singers and dancers and with some guest performers eg comedians. We love the RC shows but didn't feel we missed out.
The food is outstanding. We only ate in the buffet at breakfast and the pool grill for lunch (where they do excellent burgers) but the Main Dining room food is fantastic and on a par with RC speciality restaurants but with more choices as obviously the menu changes every night. The Gala Night dinner was one of the best I've ever had on a ship.
Drinks are all included and top quality with plenty of choices and proper champagne- not the RC Chandon cheap version. You can even order caviar for free any time of day.
We had a balcony stateroom which was similar in size to a RC Junior suite but much better appointed and also comes with a stocked bar.
The ships are small which means they can get into ports which the larger ships can't, which is particularly nice in the Med where some of the ports are a long way from the attractions.
What an amazing opportunity for your relatives. I would grab it with both hands π