You cannot apply the 2 expansions to different accounts via the bundle. The bundle for PoF + HoT is one key and can’t be used separately. That said the only time the key is automatically added to an account is if you buy it through the in game store. If purchased in the web store (other authorized retailers) the key is emailed to you and you have to add it to your account via the “My Account” page.
To gift buy it from the GW2 web store or other authorized retailer (when available) and once you get the game key provide it to your friend. Just don’t register it and don’t buy it through the in game store as the in game store automatically adds it to your account if you don’t have it already. Of course if you already have PoF on your account the in game store won’t be an option anyway.
Do not log into their account to do so. That is a very bad idea for many reasons.
Lastly, welcome to the forum and your first post. It can be a bit weird here at times but don’t be a stranger. 
(edited by JustTrogdor.7892)