Well, can I vote for your brother mis-remembering?
The only other possibility of spells and armor that I can think of are the armor rules as to what spells you are allowed to cast, and/or the armor ESF rules.
Perhaps the "fire" aspect of his memory came from the spell he was trying to cast when he had the problem/issue, and didn't really have to do with fire at all, but the casting of the spell. Those ESF rules (which are optional) might possibly have been an issue, or maybe he didn't understand the BAR attack table (the columns being based on the target's armor, not the casters).
In any case, spells resolve in one of three manners...
1) OB/DB -- no real difference from swinging a sword. Perhaps a couple of extra modifiers..
2) RR -- no armor interaction here at all.
3) Other -- if you don't fumble, it is cast pretty much. Now the effects (i.e. like the crit from a Firewall) can affect things later, but again, there is no core armor interaction on these types of spells.
As for "using all of the core rules", you do realize that a good chunk of Character Law was optional rules, don't you? The RMC Team were all shaking their heads as they revamped things and there were a number of "I thought that was core" comments among them.
And nothing in the Companions was ever considered core, not even Rolemaster Companion II.