Using a single intermediary address between your main wallet and your friend's wallet does almost nothing to protect your privacy. If you want to move coins from one place to another while obscuring the history, then you should do so via a mixer. Further, using a "temporary wallet" is a terrible idea. What if your friend decides to send some coins back to your address, only for you to find you no longer have access to that address because it was part of your "temporary wallet" which you have since deleted?
Mixer may be more effective, and more concise. I have never used a Mixer but it looks like the mixer is able to scramble your transaction address, if a see on several articel about mixer. So he doesn't need to create a new address, save the prifat key and seed. It may be a risk like you said, but I think I have several alternative wallets to store assets. Another problem is the cost, there will be additional costs if you have assets on the main wallet and sent on the second wallet that is used to send to your brother. Sometimes if busy network also needs more cost.