The FI was not sure the student could not safely complete the flight solo with the weather, so he told him to follow him in formation. The problem is the student is still alone in the helicopter, with a much higher work load than he is use to (Weather & formation flying)
If there is any doubt about the safe conduct of a SOLO flight (The weather in this case) then the student should not be sent solo. There is too much risk involved.
I see a real big difference with a dual flight, on a dual flight the instructor is there to bail the student out (Help prevent inadvertent IMC or make sure the there is enough carb heat etc)