Draw a diagram first.
Setup a coordiante system. Make the (0,0) the circle's center. Make (200,0) the position of the friend.
Can you write the equations for the runner's position on the circle over time? Something like x(t) = 100cos(At), y(t) = 100sin(At), but you have to find A so that the speed is 7m/s
Now can you write the equation for the distance between the runner and the friend over time? Call this function dist(t)
When is the distance 200? dist(t) = 200
How fast is the distance changing at this time?