Can I give HOT to my friend?
Yes you can as long as you don’t try to do it through the in game gem store. If you buy it through the online web store you will receive the serial number in an email with instructions on how to register. You can then send that email to your friend and allow him to register the upgrade.
The Burninator
Gifting Heart of Thorns to someone else is NOT possible through the in-game purchase.
However, it IS possible through buy.guildwars2.com (ArenaNet’s official store).