BMX's are a tricky subject.
If you want to go over to a smaller front ring you need a 3 piece crank which works out a bit pricey to say the least, the drilling that secures the ring is different as well. The real reason they run the smaller ring is to avoid damage to the ring on street courses and parks or on the coping of skate ramps, concrete and steel can be unkind if you get hung up on it.
If you go for the smaller ring then you have to make the switch over to a much smaller sprocket as well which again is a different system which ironically runs on an over sized axle that won't drop into a bog standard frame. The sprocket is similar to a cassette in design and has more engagement pawls, this is what creates the whirring sound you happened to hear.
The gear ratio isn't that silly at all, the smaller ring runs with a much smaller sprocket so you get what you lost in gear inches with the smaller front ring.
I would not go mad trying to upgrade the existing bike, ride it into the ground on the existing components and if he really wants to get into using it to rip around the park on then go for a new bike with the 3 piece cranks and the low profile drive train.
As for crank length, you only really need the 170's if you are on an actual BMX track with the run in ramp and start gates, it's a way of getting more welly into the first corner as the first person in to that corner tends to win the entire race.