depends what the lady in question wants to use her phone for….
If it’s voice calls and texting GiffGaff is possibly the cheapest at the expense of terrible sound quality on wide areas of the network where half-rate codec is used. It’s also worth looking at the 321 pay as you go product on Three – 3 per min for calls, 2p per text, 1p per Mb data).
If your wife wants to use her phone for the web / data, the coverage areas of the main networks are as follows:
4G
EE – 70%
Three – 35%
O2 and Vodafone 25-30%
3G
EE – 98.7%
Three – 98%
o2 – 91%
Vodafone – 90%
Root Metrics UK provides a lot of independently collected and crown sourced data on coverage / call quality and is worth a look.