If your friend was not paying attention to notice the gift text, or he didn't realize gift text devalued unusuals, then it's entirely his own responsibility. Unless there is some manner of manipulation or deceit, unbalanced trades or trades involving gifted unusuals do not generally qualify as scams or sharks.

I could not authoritatively tell you how much less a gifted unusual is worth, especially because I tend to avoid them myself, but I've heard the value of affected hats is often underestimated because of lowballers. What I would normally do is look at other sales/offers for the hat, and undercut them by about 10% or so; eventually someone who wants that particular hat will come to you because it's the cheapest, and not particularly care about the gift text. I would just undercut the lowest price by about a couple keys if you're trying to get rid of it; eventually someone will come to you because yours is the cheapest, and they won't really care about gift text.

That is excellent advice, thank you~ <3