It's sad news, we thought that using a non-custodial wallet is safe, in fact, there's no really safe over the internet.
It's safe to use a reputable open-source non-custodial wallet and that's have been said many times in many threads in this forum. It's not that any non-custodial wallet is recommended.
Transferring their fund by importing the 12 words into other wallets like Electrum might be a good step or any wallets that support importing BIP39 seed phrases.
You don't make your wallet more secure, just with importing your seed phrase into a good wallet. You should create a new wallet and send all the fund to that.