Friend recruitment question

My apologies, I wasn't sure the correct forum to ask this on.
I wanted to find the best course of action. I was going to recruit a friend so we get the benefits but as a birthday gift I know they wanted the game and expansion (collector's digital editions for the mounts.). Would they be able to upgrade to the Realm Reborn collector's edition and should I just buy Heavensward Collector's edition for them? I wasn't sure the best course of action when using the recruit a friend campaign. I don't wanna buy something twice essentially. Sorry if this is confusing. I'm not quite sure how to ask.

If you bought the standard edition of ARR you can always upgrade it later on through the Mogstation, of course by paying something extra for it, I'm pretty sure the same applies for heavensward.

It doesn't matter to the Recruit A Friend program whether your friend's game is standard or collector's edition. As Ilona said, you can get the Standard Edition first and then do a digital upgrade to Collector's Edition later, but if you know in advance that your friend is going to want the CE mounts, why not just get the CE to start with.