Oh Lordy - this really is quite a simple question for anyone in the building trade.
Insulating with silver foil is only useful for preventing partial ingress of UV light, which creates heat inside the conservatory - so it helps prevent excessively hot temperatures in a South or West-facing conservatory in the summer.
Insulating with a boarding system acts in much the same way as insulating a flat-roof with slab materials - it not only prevents the ingress of heat in the summer, but more importantly it prevents some heat getting out through the translucent roof if the conservatory is to be anything more than a freezing box-room in the winter.
To my way of thinking it is well-worth doing, HOWEVER you and the client need to think about the reduction in light caused by a non-translucent material in both the conservatory and (more importantly) any ancilliary room internal to the conservatory. With many conservatories being built onto existing patio doors, failure to consider this may result in very low natural light levels in the room beyond - it depends on the configuration.