Why? He's always riding on the sidewalk.
It may seem counter-intuitive at first. After all, it's safer walking on the sidewalk, and you're separated from motor vehicles by the curb and often a line of parked cars, doesn't that mean you're safer off the street?
Nope. In fact, a report from the 1990s showed that you're nearly twice as likely to have an accident riding on the sidewalk as on the street, and if you're riding against the direction of street traffic on the sidewalk, you can double that, making you about four times as likely to get hit by a car. Other research has borne that out, showing that two major factors in bicycle/car collisions are riding on the sidewalk and riding against the flow of traffic.
The main reason for this is the very thing that makes the novice rider feel safer on the sidewalk: isolation. While you may be protected from cars while you're tooling along on the pavement, you're also pretty much invisible to drivers, even attentive ones, who are focused on the road. You're screened by both obstacles (parked cars) and expectations (wheeled traffic is on the street, anything on the sidewalk is moving less than 4 mph).
The problem comes when you come to an intersection. As you cross the road, cars that are making a turn or approaching a stop sign are looking for cars in the roadway. If they're a good driver, they're keeping an eye out for pedestrians as well, but here you are in the space reserved for pedestrians (again, averaging about 3mph) rolling along at anywhere from 8 to 15 mph. As a driver makes the turn, you on your bike roll into the picture too quickly for them to stop and before you know it, you're sprawled across their hood, or worse, under their wheels.
This is compounded when riding against the flow of street traffic, as drivers are watching for cars traveling the other direction, and you surprise them by coming from the other one. Add to that the fact that, if the car is traveling in one direction at 20mph, and you're traveling in the other at 15mph, you're now colliding at an effective speed of 35mph, which is gonna hurt (it also reduces the amount of time you and the driver has to react, whereas if you're traveling in the same direction, the speed difference would only be 5mph, giving the car a LOT more time to not hit you).
Riding against traffic is just as dangerous in the road as it is on the sidewalk, of course. The reason most cyclists who do it give is that they want to see traffic coming, and are afraid of being hit from behind. Statistically, however, collisions from the rear are the LEAST common type of accident, whereas turning cars hitting wrong-way cyclists are an unfortunately common occurrence.
As for sidewalk riding, in addition to the danger of being hit by a car while crossing an intersection, there's a constant danger of colliding with a pedestrian, or being hit by a suddenly opened shop door (remember, due to fire codes, they ALL open outwards) or tangling with somebody's leashed dog. And while in the street cars are the big, fast and dangerous ones, if you're mixing with walkers on the sidewalk, suddenly YOU'RE the scary one.
Bicycles are vehicles, and, on 25mph residental streets anyway, move at speeds closer to a motor vehicle than to a pedestrian. They belong in the road, and in almost all circumstances are safer when ridden there (there are exceptions, such as along 50mph arterial roads with no shoulder and no sidestreets, or over open-grate bridges, but these are just that, exceptions, and if you'll have a more enjoyable ride avoiding them anyway, if that's at all possible). The safest strategy is to be visible and predictable as possible. This means being in the street, riding with the flow of traffic. It means signaling turns (especially left turns) and using lights after dark and in poor visibility conditions, and it sometimes means taking the lane for yourself and making cars wait behind you until it's safe for you to move over and let them pass.
And if some disgruntled motorist tells you to get on the sidewalk, tell 'em to stuff it, we were here first!