Make sure your transaction is signed otherwise if it is unsigned it could contain your master public key which would let your friend have all your current and future public keys (and addresses). You may not want them to know that for privacy reasons.
Otherwise when the transaction is signed, it is broadcasted to the network and everyone sees it and the transaction will be included in a block and remains there forever; so there is nothing risky about sending a signed transaction to someone else to broadcast although I have to say it makes no sense to do that.
Thanks for your good advice.
So there is no risk if I send him a signed transaction in a .txn file or a raw text.
For that purpose, best is to use Electrum on testnet, maybe multiple wallets too, get free funds from testent faucets and learn for free how the things work.
Then there's no risk at all. And if you're nice enough, after the tests you'll send back to the faucet the remaining testnet bitcoins.
Thanks for the tip.
He prefers to do all the tests with 0.001 BTC in Electrum.