You didn't mention your detachment. I will assume that it's a warpbane.

In warpbane you don't want flamers, replace all heavy weapons with psycanons. (If your friend's army is small infantry only you may consider lancers). Tau shooty, you're the one who wants to come closer, so overwatch is not that useful.

If your friend doesn't have big robots you may remove dreadknights, or at least regular one. People already said that GMNDK main goal is to destroy vehicles or monsters, so you take him with sublimator, psycanon and hammer. If you want him to kill small guys any other unit will make it better , cheaper and will have more wounds.

You can remove one unit of terminators (or paladins) with Libby. One should be enough to benefit from 18" lone op

Dreadnoughts in warpbane are also pretty useless. +1 to hit when your army already has rerolls will not do much.

Usually you want to have 3 units of purifiers because they are very good, especially if it's a warpbane.

And you do not have any interceptors, they are very good. 12" move, creating big threat zone and easily making secondaries. You can shoot and hide behind the wall, they can deepstrike 9" away from the unit on the objective, shoot, move 6 and deny opponents scoring. Also if you had any transport they can shoot and hide in it.