From the viewpoint of your student he/she can only log the time as Pu/T up until sunset. Legally you can fly until official night time even without a night rating but unless you have a night instructor rating (min 2 hours training in the air + 1 on the ground) your lesson legally ends at sunset in terms of your student logging the time as Pu/T.
Going past sunset without the night instructor rating could cause money arguments on the ground so my advice is to land by sunset unless your punter knows the score. Trial lessons are the worst scenario for this really - legally it's a no-no but 9 times out of 10 the punter isn't interested in using the time torwards a PPL so commercial pressures may mean this ruling is perhaps flouted from time to time although I have yet to witness it.
VFE.