Ok so my friend got his hands on some sort of trap that makes monsters unaffected by spell cards. Now, if he plays this while there is a monster on the field equipped with premature burial, and it becomes "unaffected" how exactly would that work? If it just stays attatched and does nothing the monster would stay because PB isnt destroyed, so would this trap instantly destroy premature? Or would it change nothing.

Or what if it made it so the monster didnt even need premature to stay alive? I dont know...thats why im asking

When a monster is unaffected by spell cards, all spell cards attached to it stay attached, but their effects just get ignored, so if Premature Burial is destroyed later, the monster stays alive.

To add onto Devil Mazera's post, just remember that being unaffected by Spells doesn't mean that the Spell card is negated. If you equip a Mask of Brutality to a monster who's unaffected by Spells, you won't get an ATK boost or a DEF minus, but you STILL have to pay 1000LP every turn.

Another question.

I got Horus LV6 on the field, with one other monster and a set card.
My opponent plays crystal abundance.

Does my opponent bring back 2 or 3 monsters?
Cos Horus unaffected by spells, he just stays there?

That is correct. Horus LV6 would not be sent to the Graveyard, so it's not counted for Crystal Abundance.

However, being unaffected by Spell Cards doesn't protect them from being the COST of Spell Cards. If your opponent has your Horus LV6 due to the effect of Mass Hypnosis, Brain Jacker, etc., and plays Enemy Controller, it is perfectly legal for your opponent to tribute the Horus LV6 for the cost of Enemy Controller's second effect.