Windfall

My mum is a pensioner and is due a windfall in a few weeks time, around £50,000.

She doesn't receive any means tested benefits, no pension credits etc. She pays full council tax and still has a small mortgage.

She does receive her state pension and a private pension.

She would like to know because she receives money from the government, her state pension, does she have to declare this windfall to them?

Thanks.