Well Im not Simon, but I think I can answer your question.
For most police services, HTA matters are always scheduled on the officers regular working days. Courts are provided with a copy of the officers schedule (including vacation days) so that they know when to book matters. Matters can be delayed or rescheduled, but the Court will typically ensure its on a day when the officer is available. Is there a chance that the officer wont show? Yes, but its not a common occurrence. Officers are legally required to attend Court, its not something that they can opt out of because they dont feel like it. If an officer fails to attend Court without good reason, they could face discipline that would typically result in a forfeiture of pay, etc.
Now there are some exceptions to scheduling matters on working days. I know Toronto Police used to (and perhaps still do) book HTA matters on officers days off to help frontline staffing. Regardless, those officers would still be required to attend Court and would be receiving the added incentive of overtime pay for doing so. Other GTA services may operate in a similar fashion.
Typically if an officer doesnt show, it would be because theyre sick or got tied up on a call for service prior to Court. Sometimes there used to be problems with officer being double booked in multiple Courts (i.e. Criminal and HTA), but I hear less and less of those issues.
Id agree with your friend that 99% of the time its just luck. If however youre asking because youve been charged with something, it might help to let us know what region/police service youre dealing with. Each is slightly different and you may have more luck in certain Courts.