If your friend is happy with one or two free tickets, paying for the rest, she should be able to achieve that without too many complications with redemption.

SPG Amex and PRG are the obvious suspects, plus the business versions of each card if desired (but the Delta Gold Business has the lowest minimum spend). Present the best offer details and links, explain how the points transfer, and suggest that she have her husband apply for them too. Which cards and when should be up to her, some people are nervous about the idea of tracking multiple new accounts that may need to be cancelled after a year, plus you don't know how many Amex accounts they already have. Start with the SPG referral and then send a email with her other options. An SPG card will get her the other half of the ticket. Two Amexes for her husband would get another free ticket. Getting more free tickets will take more time and determination.