Gunner_Sabot_Tank hit the nail on the head move out of cover then assault, even terminators would fear these.
@Darkstrider, I’ve considered your points now in great detail after playing against the best DE, E and Tau players in my LGS and have compiled both my advice and other Tyranid players’ from my LGS to answer the questions.
1) Either using Mycetic spores and deep strike to come up all around the target, or being the swarm and attack from everywhere at once using cover to block LOS. Some advice from another player was to use distractions, e.g. Carnifex’s.
2) Now he should use fast moving counter attack units to go for the weaker rear armour e.g. Flyrant, Harpy or Shrikes .Another option would be for him to use deepstrike for the close range annihilation e.g. Lictors, Raveners and Zoanthropes in spores. Some more advice would be using spore mine clusters to restrict deployment types.
3) Pretty much the same as last time using fast moving units to destroy the transports then assault to mop up the rest. No advice yet.
4) Genestealers are the best way of negating this along with Broodlords, Lictors, Raveners, Hormagaunts and my personal favourite LASH WHIPS! Other advice is psychic powers (paroxysm), killing many in shooting and sheer number of critters in combat.
5) Actually I go against the grain here a bit; go for lots of monsters and by lots I mean absolutely friggin’ lots. Failing that take 5 Tervigons in 1,000pts with 30 Termagants and another cheap unit. Advice from everyone else; “If they take poison, take a swarm”.
6) Tau only have a few weapons that ignore cover (SMS, seeker missiles, airbursting frag projector, markerlights and the multi-spectrum suit), so to counter these take them out SMS are on 5 units, seekers are dirt cheap but do little against the swarm, the airbursting frag projector is 1-per-army and has a tiny range. Now the markerlights are usually the first on everyone’s priority targeting list so to stop them go Biovores and finally the suit upgrade 1-per-army, mega expensive and the model cannot shoot, only worth while in a Farsight bomb with the same one taking the CaC node. So just take Venomthropes and swamp the enemy in units.
So a final note for your friend let him use different playstyles and allow him to figure out what is best for him. Failing that come back post another thread up add let us help you.