Well I've been having a chat your him just now.
He is worried about damaging the exterior house wall, but he thinks that drilling his own hole for the fibre cable, from outside to the inside would be the best option, because then you can just plaster over any damage on the inside wall.
I think going below the damp course is not a great idea, so hopefully he won't do that...
I've told him we should try to do it as simply as possible (not plan for future fibre cables from other providers, not planning for future fibre cable cables from other providers with a rope pulling system, and so on), so I hope he will agree.
For future ISPs, you can just use a single hole in the wall, remove any previous wall boxes etc, then just reuse the same hole...
I've done extensive renovations to every home I've had (all my own work) and drilling below the DPC isn't required to avoid damp as the DPC is just to stop rising damp. If drilling holes in walls was an issue then we would be in trouble with windows and doors (although sadly many window companies don't get this right when fitting replacement doors/windows).
All that is needed is to drill so that the hole is higher on the inside than out so there is no water ingress even without sealant into the cavity or internal walls. I mostly drill inside out although it depends on the situation and disable SDS/hammer drill (if used) before reaching the other side to avoid brick burst which ISP fitters seem to like so much .