Try to reach out to that guy (who sent the coins) and ask him about the instructions he gave to your friend. It might help to figure out what happened because knowing how the watch-only wallet was created is the key to solve this problem.

You will receive more emails/pms from newbies offering to help you. Do not trust them and ask them to post here if they want to help.

No, that's not possible.

If he generated the private key using mycelium, he definitely was told to back it up. He might have forgotten this. Or he didn't care.
If he imported an address, there was never any private key to start with.

Either way, this is a prime example on how fool proof methods can never be fool proof enough and that for the majority of non-techy people a wallet has to be as simple as using a credit card without any addresses, private keys, transactions, confirmations etc.
Basically for the mainstream user, you'd need to remove every technical aspect of bitcoin. And even then it wouldn't be fool proof enough..

In OP's specific case, the mnemonic code is kind of worthless.
At least, it is not usable to recover that one private key OP's friend is looking for.

The private key never was derived from the seed. It was added as an "additional account" (or similar) inside of Mycelium. It has nothing to do with the mnemonic code and the HD wallet itself.

Mycelium hs a feature where you can change the SA account to a watch only wallet by deleting it's private key.
Since your friend believes he created an single-address account and didn't import a public key then I suppose he mistakengly deleted its private key which converted the wallet to a watch only wallet.
Unfortunately, after deleting the private key there is no way to recover it.

If he doesn't have the private keys or recovery seed (12-word seed) to that address then there's no way he can transfer that funds from mycelium to another wallet addy.

Does your friend created the account in mycelium by the "advance" mode or "Add Bitcoin HD account"? or just imported the address there?

Having a watch only wallet means you can just watch the activities of the wallet but can not spend any coin. It's the same as watching an address in a blockchain explorer. The difference is that you can create a translation but can not broadcast it until you have the private key.