What is the best outfit for a 45 year old lady to wear to a Robbie Williams concert?

Seriously, I want to buy my mum a great outfit to wear and need some advice. This is the first concert she has been to and she is not some teeny bopper, she just loves Robbie Williams. It will be summer and hot when she goes so real tips are needed please. Oh yeah, she doesn't want to flaunt herself, just look nice. Thanks