Wow if they wait a couple of weeks, they are ganna lose out on some money...
With the new system, players will be able to link their account up with another account via the friend's list retroactively, even in the new account wasn't created WHILE linked up. Customer Support could always do this for players, but with the 5.4.1 RAF system, players will be able to retroactively link themselves in the 90 first days of the new, friend account. If your friend whats to start now, he can, and when 5.4.1 starts you can get yourselves linked up for the promotion via the friends list.
With the speed in which leveling goes now-a-days (even without BoA items), you guys will still level up pretty quickly regardless of the promotion not being applied.
Pros: You'd end up with the mount and free month, still, after your friend has subscribed for 2 months of WoW.
Cons: may end up losing out on some triple XP time, and they'll miss out on some grantable levels for you.