Eh, it was a fairly short flight. I don't think that there was really anything wrong with him for not wanting to switch for a different seat further back on the plane. I don't think that makes him insensitive. The mother could have (or had her children) speak to someone at the gate, check-in, or to a flight attendant if they absolutely needed to be seated next to each other. That said, I think in this case it would have been nice to do, but there was no reason why they had to be seated next to each other. Your friend provided an alternative and the mother did not understand and/or was unable to orchestrate it.
Personally, I probably would have been inclined to switch provided the seat was not much further back, but I do not fault your friend for not switching. There are many reasons why someone may not want to change seats. I prefer an aisle seat, but the window seat does not bother me, especially on that short of a flight. Switching would have been polite to do, but not necessary
As for the language barrier, that happens. Again, if it was that important, the woman could have had her children translate (presumably they would be able to) or involve a flight attendant. That said, if a switch needs to be made, the person who is requesting the switch should take the burden of the responsibility in making sure this is accomplished.