I'll assume you're at a Games Workshop. The LoTR series isn't as well known so even the judges won't know all the rules but generally the judge makes the call like it or not.
1. Yes but if you're playing a friendly allow him to do it one time. He won't make the same mistake twice. If its a tourney allow it and tell the judge he forfeits and your playing for fun.
2. No but same advice as above. Additionally you can ask for something in return since now he can exploit your list (what do I get)? Ask for an additional Hero, might, priority... whatever you can get.
3. Well you musta whooped him. I offer my opponent to surrender every turn but usually they fight to the last man and as do I. People get emotional sometimes when they spend all their time and money on their hobby only to have it destroyed with ease. I'd just forget about it and move on.
Also there is a rule I use and its called a Gimme. Its actually a 'Give Me' but that's how I pronounce it. Anytime you allow someone to break a rule,reroll, redo or go back in time you tell them they owe you a gimme so next time you want to correct something you just call your gimme and do it. I allow it all the time and its rarely forgotten.
I made a thread on
Wargame Etiquette that I researched awhile back if you want to give it a read.
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http://drinkingwotr.blogspot.com/ - Links to my CrAzY House Rules, Games and 'poor man's' terrain.