The possibilities are:
1/. You texted a Giffgaff number, and were charged - that's a fault which we need to report
2/. You texted a non-Giffgaff number, and were charged - that's intentional
So you need to check whether the number you texted is actually on Giffgaff. Your friend could be like me and have 2 mobile phone numbers - and I don't give out my Giffgaff number (because it's used for an important private purpose).