By now, you have moved that to a different wallet (i.e. one with a different mnemonic), right? You can assume that this private key is compromised - you cannot use any of the addresses in the BIP39 hierarchy that are rooted by this key any more. Did the other person really needed to have access to your mnemonic (i.e. in order to restore your wallet elsewhere) or was perhaps a matter of getting access to a computer or even worse a phone that was left unattended?
In any case, in order to prevent such mishaps you need a HW wallet.