Seat width on planes?

i have a friend who is overweight, she is scared that she will need a seatbelt extension on her next flight which will be with thomas cook. she has flown with ryanair before and she was fine and didnt need one. then flew with easy jet and was fine but she was uncomfortable, can anyone tell me the diffrence between easyjet width and thomas cook width of their seats??, thanks in advance. no horrible comments please as it will just be reported.