Weapons should break when its appropriate for them to break. Headbites can punch through steel so blocking a headbite can potentially break any weapon. I would say 50/50 chance the weapon breaks.
Although if a player blocks a headbite with a complex weapon like a pulse rifle and it breaks I would downgrade it to a blunt instrument. If a player blocks with a blunt instrument and it breaks it gets destroyed.
That helps avoid the situation where your players are totally unarmed against aliens that get to go twice in a turn.
At the same time, that same player's block could be described as pushing the Xeno's head to the side with the weapon, causing the headbite to snap at empty air.
I don't think it's fair to just assume that the action plays out in the way that penalizes the players the most. Since the blocking rules don't
specify item breakage, I think it's more reasonable to assume that every time a block succeeds, it's doing so in a way that makes sense
: if we know a headbite can punch through steel, but a block succeeds... clearly, the player wasn't shoving his gun in the alien's mouth. He must have been blocking another way.
The danger you run into is being (unintentionally) arbitrary or inconsistent. If the players expect the rules to work the way the book says they work, and you change your mind in one instance but not the other, that can get pretty unfair. You should at least be clear well in advance that you intend to change or modify a rule, and the players should be clear on how that rule was changed or modified, so they don't feel cheated when the xeno bites through their shiny pulse rifle despite a successful blocking action.
That said, sometimes you need to bend things and surprise them to build a powerful scene. Just use discretion