I was recommending use of online wallet as-
1. He can directly but, sell Bitcoins there
The two online wallets you mentioned has very specific policies regarding transactions related to gambling. If they see any transactions that involves a gambling site, they may close your account without any notice.
If I'm right, Xapo is based in Switzerland. So, it does not restrict gambling transactions.
But yes, you are right for Coinbase(US based), it has closed several accounts for gambling site transactions.
Personally, I feel UI of Online wallets is good.
3. He is only willing to use it for a specific thing. ( Maybe to be used only for done days). So why download the application.
I'm not sure how hard it is to take 2 minutes to install a program as opposed to creating the account in 5 minutes. Desktop wallets gives you a lot more privacy and security.
Yes, you are correct that it can take more time for setting up online wallet.
But security isn't a concern of online wallets.(Only concern is the wallet company scams; Coinbase Xapo are trusted) We have an option to set 2FA + password. Also, while making any transaction, your IP would not be revealed at the blockchain.
6. It would be easier to remember passwords of Coinbase or Xapo.
Does anyone still memorise their private keys?
NO, I'm not telling about memorising the key.
He may misplace his private key.
Yes, but that's not a big point.