His choice how he drives his bike at the end of the day.
He's on a moped, it was wet, bikes skid out easier when in the wet particularly if he's moving off the line quickly and turning to get in between two cars.
Maybe he, or a friend, or family member had an accident earlier? Maybe he had a near miss prior to this incident involving another vehicle which made him take things a bit steadier?
Perhaps understand and accept that not everyone drives the same, beeping them because they aren't moving or acting as quickly as you like if just ridiculous, people learn to have some patience sometimes.
This is coming from someone who doesn't even ride a bike