at someone's expense
US /æt ˈsʌm.wʌnz ɪkˈspɛns/
UK /æt ˈsʌm.wʌnz ɪkˈspɛns/

1.
auf Kosten von jemandem, zu Lasten von jemandem
paid for by someone; causing loss or damage to someone
:
•
The company party was held at the company's expense.
Die Firmenfeier fand auf Kosten der Firma statt.
•
He made a joke at her expense, which made her feel uncomfortable.
Er machte einen Witz auf ihre Kosten, was sie unangenehm berührte.