You could always buy them a voucher, You can purchase a giffgaff voucher from most supermarkets, corner shops, newsagents, petrol stations & Post Offices as well as loads of other stores who provide epay, Payzone or PayPoint services.
For more details see this link https://community.giffgaff.com/t5/Solve-your-questions/Where-can-I-buy-giffgaff-top-up-vouchers/ta-p... and this link https://giffgaff.com/support/top-up-and-balance/using-vouchers
I have a friend who is with Giffgaff and is struggling to top up her phone due to money problems. Is there anyway I as a non Giffgaff user can put some credit on her phone. I have a PayPal account and cards
You could gift your friend a voucher
£10 minimum see