It's my experience in the state of California (Northern Bay Area) that radar detectors do very little when you're on the freeway. Police officers don't rely on radar on the freeway that much and will usually just sneak up on you and pace you.

Radar detectors are usually most helpful in the suburbs where the typical posted speed limit is something like 35-40 mph and you're cruising about 5-10 mph OVER that limit. Afterall, it's really easy to do 45 mph in a 35 mph zone and not really notice. In a situation like this, radar traps are usually used and a radar detector is usually you're best defense against such situations.

In your friend's case doing 90 mph in 65 mph zone (I'm assuming freeway), he was probably paced. In the 35 mph zone doing 54 mph, probably laser.

By the way, doing 54 mph in a 35 mph is not cool at all. Tell your friend to slow down.