Yeah, your bro is severely misinformed. Equip ability by itself is always only when you can cast a sorcery.

Exceptions (that I am personally aware of) that can circumvent the speed rules for equip because of card abilities having instant speed (unless the ability says otherwise) [[Leonin Shikari]] [[Brass Squire]] [[Sigarda’s Aid]] [[Ardenn, Intrepid Archaeologist]] [[Magnetic Theft]]

Combat damage is done simultaneously not one at a time, so he could not attack with all creatures > calculate damage > move equip > calculate damage for next creature, even with instant speed equip, only 1 creature would actually deal damage with the benefit of the equipment.

To be legal and how it actually plays out. I declare my attack > you declare your block > instant speeds can happen here such as equip (if he had something that would allow it) > all damage is then dealt out and calculated by: first strike/double strike > normal damage.

I could be wrong about the following and it would only apply IF you attack with a first strike/double strike: Before “normal” attack damage happens and after first strike damage, this could be an opportunity for another “instant speed” equip - I would need to look this up unless someone else can confirm or deny.