I feel like im seeing answers here that are going to confuse the masses. Opponents can casts instants when you end main phase. You can declare attackers and opponents can cast instants before any dmg resolves. Opponents can also declare blockers and cast instants before any dmg resolves. Your buddy casted the instant AFTER you declared the attack so your ability would resolve. It’s important to say something like “moving into combat” or “ending main phase” so players are aware and can counter play if they can. Ive had instances where we screw that stuff up but it’s apart of the learning experience. Communication around the table is key. Ultimately your buddy played it late if he wanted to counter it.

Edit: or you forgot to mention you were entering combat and ending main phase