I don't know how to tau too much either, but i have played almost too many games against them, lol.
Here's my two cents:
The crisis suits can have drones with them so he can feel free to proxy them for whichever one he needs them to be
Seeker missiles are his friends, so the piranha could take a couple to make use of the marker lights.
If you didn't know, markerlights are what separates tau from other armies, they're a built-in tool box full of options
Idk what its called, but the oathfinders can also take some crazy gun that has gets hot that can do pretty okay against most things.
The main thing he'll need is fire warriors, they make up the bulk of his troop choices and have decent wargear on them.
Crusis suits are generally tooled out with a specific purpose, whether it be all the melta or flamers etc, try to get him to keep his suits uniform in that they should take all the same spec weapon, a lot of people forget that more of the same dakka goes a long way and it keeps him from having a suit ir two from sitting around not shooting for a turn.
Basically, he should just gunline the rire warriors, pathfinders and if he gets one, the hammerhead. And use the suits where they need to go.
If he wants a hammerhead, tell him ti get one AND a devilfish. As they run in the same chassis, he can build the skyray turret and throw it in the devilfish and voila, he has two heavy support slots filled and he can use the markerlights to have the skyray alpha stike with a ton of missiles.
As for you: don't worry about crushing him in his first game, something i was told when i started was to not expect to win until after my 12th game. He needs to learn his codex in and out, on and off the table and what everything is capable of. Don't go full throttle on him, but make a list that has definite strength to it to show him how to use his points to their potential. Help him iut if he has questions, and introduce him to 1d4chan.com, they have general tactics on every unit in the codex and provide a list of thei unit's strengths and weaknesses.
Other than that, just remind him that its a game and that he needn't get down on himself for losing a few times when he starts out.
Have fun, hope this helps,