Whatever Frank heard or saw and why he is't testifying is all very mysterious it would seem. It's the curious case of the gardener in the nighttime really isn't it. But of all the people involved in this, he is the one probably at the most disadvantage so I feel for him. Not a South African, possibly not as educated as the neighbours, employed by the golden boy of a wealthy, powerful established family, probably questioned from all directions - lots of pressure that could well have been very intimidating.
If anyone really knows what Frank heard and saw it may well be any friends he has on the estate who are also live-in employees. Anyway, hopefully his testimony will not make a difference either way and the guy can live in peace. At least, as someone funnily said earlier, so far OP hasn't blamed him for it.