On most Android devices a screenshot can be taken by holding the volume down button and the power button at the same time.
Samsung devices are slightly different, a screenshot is taken by holding the power button and home button at the same.
The screenshot will then be saved in the gallery from where the customer can share or send it by email.
A screenshot can be taken by first holding the sleep/wake button then pressing the home button.
The screenshot can be found by going to the Photos app > Albums and tap Camera Roll