Missing Bitcoin

Hi, just looking for some advice.

my mum sold some of her bitcoin on blockchain, and all her bitcoin is now missing from her DeFi wallet. She has withdrawn the money from the sale (about £1000 worth), but there should have still been some bitcoin remaining (about £700 worth).

I checked the transaction details, and it's showing sale of just the £1000 worth, but her DeFi transactions show all the BTC moving from her wallet.

Please can someone advise why this has happened?