Yes, you are right. The seed is all you need to recover your wallet so make sure not to lose it.
Note: blue wallet afaik follows the bip39 standard to generate seeds while electrum uses its own standard. So, you can't use blue wallet to recover a wallet generated by electrum from seed.
The cold storage wallet is on Bluewallet, while the watch-only is on Electrum. Bluewallet is BIP39. It can be recovered easily on Bluewallet, or if want to use Electrum, the BIP39 box will be checked if importing seed phrase on it.
There's just a requirement. The wallet must have been funded. If there is no used address in the wallet generated by electrum, you can't import it into bluewallet.
I imported a seed phrase from Electrum wallet to Bluewallet, any of the address of the Electrum wallet has not received any coin before, they both have the same keys and addresses, Bluewallet generated it without any address funded before importing the seed phrase.
This is just asking for trouble. For fool proof operation you should use the same software on both online and offline devices. I suggest you buy a used desktop or laptop instead and install electrum on that.
I too will consider this a good step to follow. But making sure he reinstall the OS himself and make it airgapped. I do not know why he wants to make use of two wallets when Electrum is perfect for both cold storage and watch-only.