Front guard I would think is regarded as part of the structure of yhe forks as it also braces so unless he's replaced that strength I would expect it to fail. Seperate issue is construction and use regs, again, as far as I am aware there should be a front gaurd and there is a minimum coverage area. I don't get the deliberate rat bike look. I don't have phots around I think but my cb550/4 was a true rat bike, mostly totally standard, run on sod all budgett, the missing side pannel was replaced by a cb350 one held on by a zip tie. Rear guard was a steel cheap custom item bits that got rusty were painted. That 750 is mostly just an unfinished bike.