Preferred seating is for Medallion members and you have to fly to get Medallion Status. There are credit cards that will give you Medallion points, but you would have to spend a lot to get Medallion status that way and I don't think that would work for you in this situation.

Your friend can buy her ticket now and if there are no seat assignments available now, she can get her seat at the airport. No seat assignments available is not the same as no seats available (flight sold out). I wouldn't suggest that she wait until the day of departure to buy the ticket as the price could be quite high then or there might not be any seats available.

Bottom line as as long as the flights are for sale, you can buy a ticket. Your friend may not get a seat next to you, but she should get a seat, even if it is not assigned at the time of ticketing.

Hope this answers your questions.