The answer, of course, is "as little as possible."
Fortunately, with the data provided in today's strip, readers need only assume the size and luminousity of the star to work things out for themselves. Unfortunately, they may also have enough motivation to do this math, since the author is going to offer sketches to the first five answers deemed correct by his panel of judges.
Assume that the star is identical to Sol - it weighs 1.9 x1030 kilograms, and puts out 3.8 x1026 joules/sec. At the current rate of deceleration, the sail-ship will approach to within 90 million miles of the star in exactly eight months. The sail is 102,000 kilometers in diameter, and only an insignificant portion of that surface is in shadow. When it crosses the center of the system (assume a mercurially-close passage), the sail-ship will have exceeded stellar escape velocity by a factor of three.
Given this information, you ought to be able to determine the current velocity and mass of the sail-ship, as well as its current distance from the star.
You also ought to be able to resist the temptation to spend this much time on math. After all, the author did, and you're certainly a better person than he is, right?