So, here's what happened:

1. My friend asked me to back him up and sell this guy 80 BTC. he sends me those 80 BTC because I didn't have enough in my balance.
2. I meet the buyer and do the trade, I send him 80 BTC.
3. By nighttime, the transaction disappears from my wallet and the buyer's wallet.
4. Turns out the transaction my friend sent me was never processed. So he had those 80 BTC returned to his wallet. But for some reason they still showed up in my wallet during the trade, that's why I didn't hesitate.
5. My friend just sent me those 80 BTC again. Which I will froward to the buyer. All sorted out.

Still, this is a bit frustrating and the blockchain.info wallet should be improved to prevent such things, in my opinion.