What happen to A and B now?
That depends. If it was just a normal transaction with out any attempt to scam, then everything happens as usual. (Blocks are orphaned every day. It's never been a problem, the transactions just eventually end up in blocks that aren't orphaned)
B gave products / fiat to A, then now B lost all the bitcoins it received from A? or
That depends.
If A knew about the fork, and new that B was looking at the wrong fork, and A was technically skilled enough, then A could have sent a transaction to the fork that B was using, and sent a different transaction that used the exact same bitcoins to the other fork. A would know that B would see some confirmations and that B would send the fiat, service, or product. Then eventually the fork that B was using would be orphaned, and the transaction would no longer be valid because of the other transaction that A got confirmed on the valid fork.
If A did not attempt to get conflicting transactions confirmed in each fork, then A would just send the 1 transaction to the bitcoin network. The transaction would be confirmed in both forks. Initially B would see it on whatever fork he was using. If it turns out that B was using the wrong fork, then when that fork is orphaned B would see the transaction in the valid fork and might see the number of confirmations on his transaction change.
B told A "sorry your bitcoins are lost in block orphaned therefore i won't give you any products or fiat"
That depends.
If the transaction is confirmed in both forks then B will see the transaction in the fork that is not orphaned and B can then send the products, fiat, or service.
If the transaction is only confirmed in the orphaned block, then B will wait until the transaction gets confirmed in the valid fork. Then he can send the products, fiat, or service.
