TK is not 100% at fault here
I find it hard to believe that "experienced" travelers will not notice this mistake. They probably said that to save face. And if they didn't, they are definitely not "experienced" travelers. The first thing I do when I decide to go to a new place is look up the airport code - it's because I usually book my own tickets instead of having someone to do it for me. But let's say they didn't do that. After receiving the confirmation e-mail, they should have checked everything was in order - the dates, flights were the ones they wanted, obviously the departure and destination as well. They failed to do that. Furthermore, I'm surprised this was not caught at check-in at the airport. I'm guessing the visa requirements for both are different. Wouldn't a check-in agent have caught that the couple doesn't have visas for DAC? Unless they didn't require any, which makes it even more diabolic. Furthermore, once obtaining the boarding passes, the destination is clearly spelled out (at least on TK boarding passes). If you see Dhaka as opposed to Dakar, why wouldn't they ask about it? This could have been caught at LAX upon departure if not sooner.
Whereas this can be attributed to "lost in translation" and TK ended up issuing tickets to the wrong destination, there's absolutely no excuse for the couple not to check on what they've booked, especially, when flying to a foreign country - one apparently they've never been to previously. Since they didn't recognize the mistake, how is the airline supposed to? Once brought to TK's attention, they did the right thing and flew them to Dakar free of charge and upon further inquiries, ended up receiving 2 free tickets. The gull of that couple to place the entire blame on Turkish airlines. They should have been happy that they were transported to their destination free of charge after not catching the mistake earlier themselves.
I have experienced a somewhat similar situation twice. Since I always book my own tickets, I once booked a ticket from the wrong city - Sydney instead of Brisbane. Since I watch like a hawk for the e-mail confirming my ticket, within half an hour of booking, I realized upon receipt I booked the wrong city and called to correct it. Since it was an Australian trip and they don't have the 24-hour free cancellation enjoyed in the US, I was on the hook for the cancellation fee but was able to use the remaining balance for a new ticket with the correct origin and destination this time. The second time, the origin and destination were correct but the date was wrong - I had used Kayak to find the fare and since I used flexible days, I totally forgot that the lowest fare I selected may not be on the date I wanted. Once again, I caught my mistake within the 24-hour window when I called the airline to obtain my other confirmation code (the flight was operated by another airline). That's when the agent informed me the date was not what I wanted and I changed it. So, I have been somewhat unlucky when booking flights on Australian carriers. But bottom line, one has to be extremely careful when booking through a 3rd party - I avoid that as I like to see what I'm booking myself as opposed to someone doing it for me. A lot of airlines don't make available inventory by partners for awards online, so I have to call and have an agent to do it. Well, as soon as the call is over, I'm making sure the reservation is what I wanted. Clearly, mistakes happen, so one has to be very cautious and inspect everything. The couple didn't do their due diligence here, so I would place more of the blame on them in this case. And I think TK was more than accommodating. The certainly corrected their mistake and I don't understand what more the couple wanted - they are very ungreatful.