Your friend doesn't know what he's talking about.
You can make any mod you want to as long as it is safe going down the hiway. Safe, means legal in height, width, length, and weight also. And your insurance co couldn't care less,....until it was ever proven that your mod is what caused an accident. And even then, they would have to pay off, unless it was proven that you somehow committed fraud to get the ins co to pay off.
As far as any codes, OEM's are bound by certain codes, but you are not. That's why an OEM isn't allowed to install an unvented heater, but you are. And they must install a CO detector, but no law says you can't disable it, or throw it out, etc etc.
Rich
'01 31' Rexall Vision, Generac 5.5k, 1000 watt Honda, PD 9245 conv, 300 watts Solar, 150 watt inv, 2 Cos 6v batts, ammeters, led voltmeters all over the place, KD/sat, 2 Oly Cat heaters w/ ox, and towing a 2012 Liberty, Lowe bass boat, or a Kawi Mule.