Steam gift disappeared?

maybe he clicked "add to my games library" instead of "store in my inventory"?

When you send a gift to someone, receiving user has the choice to add to his library, or to add to his inventory. In that second case, nothing forbid to send the gift to someone else immediatly.

I sent it and received it myself though, I sent it via the Steam app and received it on their account via Google Chrome, added to inventory.

It happens to me too. Move any gift in inventory "send as gift to friend any game" and "dissapeard" games show you on acc. And u can cancel sended "rapair" gift... btw sorry for english

Ok, I've gone through Steam support to try and find out what's happened and came back with this message:
"A gift sent to your account has been removed due to a billing issue with the original purchase. Your account will not be suspended, however the game you redeemed/unpacked to your account will be removed."

Is it safe to assume that whoever I got the game from has ripped me off?
From: https://support.steampowered.com/kb_article.php?ref=5854-QWLN-2621

Have you tried restarting Steam (the client itself, not just a browser).
Also give it some more time. Steam was goin down several times for me in the last hour, it should appear later (after re-logging).
I think you should see a warning message popping up if a game got removed because a faulty purchase.
Also, are you sure the game was not redeemed by the account? Checked the account details page if the license is not there?