suddenly demanded her PUK (I am not going to go into why, but she was in London)
Erm, I'm going to have to ask why. Why should the fact she was in London be related to the fact she was suddenly asked for a PUK ?
Why ? Most of the carriers have an automated PUK line you can call. No need to decrease security by writing it down. Writing down your PIN/PUK is about as silly as writing down your credit card PIN, especially if you carry the written note on your person.