Howdy. Have your friend take a look at the Eibach Pro Truck Sport shocks. They are about the same price as the Bilsteins but were specifically designed for Avalanches and offroad performance. My Av was the project vehicle - so I provided all of the info and we had to have dyno'd the initial shocks 50 times before they were happy with the performance. The shocks worked extremely well on my Av - we just did a big Death Valley trip and while others were struggling along at 15-20 mph in the rough washboard - I was sailing along at 60mph.
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Having said that, actually just changed over to some custom Fox shocks and Hummer springs as I'm adding a lot of weight and going longer distances. So, went with a Fox 2.0 outer, but with 2.5 inner and a 7/8" shaft, custom valving and bigger external adjustable reservoir/bypass. Seriously slick - but a much higher price point.
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Pic with the 35"s, leveled, and the Eibach shocks
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For brakes - definitely look at upgrading - I now have the Power Stop Z36 Severe Duty kit for the GMT 900 (1" bigger) and it makes a huge difference. Moreover, learn how to disconnect the ABS - that makes an even bigger difference.
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For tires - have him check out the General Tire Grabber x3. This is a seriously bad to the bone tire. I've had it in everything and it grips like crazy, and is relatively quiet on the highway. I will be buying these again. I had 33" on mine and used CST keys to level my front end (his 07 will have front coils rather than T-Bars - I'm jealous). But, leveling, he should be able to run a 33" without issue as long as he runs a 0 offset wheel. When I went to the 35" tires - everyone told me I had to do a 6.5" lift - I didn't want that - so I trimmed my fenderwells and part of the firewall valance - and the 35"s fit like a charm - no rubbing at all.
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One of the best things your friend can do is to invest in some Rare Parts 1.5" tie rods. The dinky 1/2" tie rods from GM are a serious weakpoint. Adding bigger tires and going into the dirt will prove that out pretty quickly.
Ping me with any questions.
