Adding a further note of caution here...
If the instructor is NOT night instructor qualified (i.e. has night restrictions on the FI rating) then the lesson MUST end at the start of official night.
however...You CANNOT have the student then PAY for the content of the flight AFTER the start of night - except under a flight which would be private - because otherwise your flight has become public transport or aerial work depending on how you look at (probably aerial work as a pleasure flight). The night portion would therefore have to be shared as a private flight with the cost borne as appropriate in the ANO - and that means the instructor must pay his fair share.
This does however if you accept the above then become a question of whether a flight can start off as aerial work and become a private flight - personnally I think not. If it starts as aerial work in - then AFAIAC it ends as aerial work - in which case then instructor has just put himself in a position where he is not legally entitled to act as P1, as he would be deemed to be providing instruction.
Now if you want to risk that debate with the authorities/your CFI...go ahead. Personally - get the night restriction removed and dont be silly enough to venture into these somewhat muddy waters.