Passengers should not engage each other on contentious issues. That is why there are FA's. They are trained to deal with controversy and more importantly, in the case of an airline employee, are even more likely to have an impact. The fact is that OP and colleague were rude and insensitive. If they needed the time to speak on a business matter, they should have planned that better,

But, that does not justify anything the other passenger, whether an off-duty employee or not, did. If he was disturbed, he should have asked a FA to speak with OP and colleague, presumably OP would have realized that he was disturbing others and would have piped down.