That was very kind of that passenger, but he or she had no obligation. It was likely offered in response to your fit.
Before/in placing of throwing a fit over airline seating, there are many steps that can be taken:
Be flexible - don't expect your entire party of 80 to sit together.
Work with the airline's telephone Customer Service Department for a solution, such as cccdisney was able to do.
If that doesn't work, check in for your flight online EXACTLY 24 hours prior to departure; many airlines open up previously blocked seats, or upgrade Elite passengers - thus making more seats available for choice.
If that doesn't work, arrive at the airport EARLY. Whether you use curbside check-in or go to the counter - and yes, you NEED to do this even if you already checked in online - again appeal for assistance.
If that doesn't work, talk to the Gate Agent.
If that doesn't work, talk to the Flight Attendants.
Using ALL these steps, if necessary, should pretty much eliminate the need for "pitching a fit".