Is your credit going down, or is "his" credit going down? Consider the various possibilities.
Goodybags don't cover all services, so credit is often needed.
Some chargeable examples are sending MMS (16p), dialling 084**/087** numbers (12p/min), short text codes, international calls/texts and phone use outside the UK.
A common problem is texts being converted to MMS - group texts, long texts ( sometimes even as low as 160 characters ) and some emoticons cause this problem
If goodybag allowances are used up early, credit is required for further use (eg 10p/min for calls).
See this Knowledge Base article about non-goodybag charges -http://bit.ly/kbggwhycredit
Here's the UK price listhttp://giffgaff.com/index/pricing