A few misconceptions, here:
1) If the works did not fall within the scope of the Building Regulations, (i.e. they were not Building Works, as defined in those Reg's), my understanding is that no breach of the Regulations would have occurred, even though they may not be in accordance with the relevant Approved Document guidance;
2) It is not true to say that non-fire protected downlighters will automatically reduce the fire resistance of floors below that recommended for domestic properties. TRADA did some tests that demonstrated that integrity, insulation and loadbearing capacity of a well-constructed floor was not unacceptably reduced (details are available on their website);
3) Having said all this, if I were the original enquirer I would consider pulling up a floorboard to see whether the cavity is full of cr*p and, if it is, see if the neighbour would be amenable to getting it cleared out. If he's worried about the gaps between the floorboards, he could fill them in himself... and, of course, fit smoke alarms, if they're not there already.