I just hope that his goal is not to create a seed with Trezor, and then use it for desktop/mobile wallets. Some people usually do this because they want to have the same wallet on their computer and smartphone, which is of course completely wrong and risky. That's why we have cases where people claim that someone hacked their HW, only to find out later that they saved their seed in the cloud/e-mail or that they used it in some other wallets.

Unless there is no other choice, seed generated in a secure environment should not be entered into any desktop/mobile wallet, and especially not into any online wallet.