But there is more to a professional alignment than just toe measurements, more more than just toe can cause a pull. Professional alignment systems also measure caster (front and rear), camber (front and rear), thrust angle, and rear toe. You'll have a hard time getting all these measurements within factory specifications using a string or long bar. I'm not trying to say you can't get by doing this, if you are starting with a straight car. However, in this case, I think the owner would be best off to get the car checked by a professional.
At the very least, he'll find out that the car IS straight and can then focus his efforts on other options. And, he can apply your methods in the future and be reasonably certain the results will be reasonably accurate.
Besides, a professional alignment, if it the alignment is not off by much, will cost him less than a set of tires and hopefully prevent him from needing to replace tires prematurely.
-Rod