The vast majority of calibration technology requires both a colorimeter and appropriate software. There is plenty of good quality calibration hardware and software available out there for under $100. Maybe not the most exciting purchase you've ever made, but a necessary one.
There is also software which allows you to do a visual calibration. These should not be trusted, especially for working with clients. The eyes play even more tricks on you than the monitors, and incorrect calibration is an incredibly easy mistake to make, especially if you're vision is less than perfect like most of the world.
When deciding what calibration technology to purchase, make sure you get something flexible and mobile, preferably one that will work with a variety of operating systems to fit every client's need.
If you don't have any calibration technology when you discover the problem with the client, you might not be entirely out of luck. With a little fiddling, most monitors, especially older ones, have the ability to reset to their normal value—which might not be perfectly calibrated either, but it's a step in the right direction.