[Inziladun]

My assumption has been that the Dunlendings learned Saruman had largely lied about the Rohirrim, but he had also enlarged upon historical grudges the Dunlendings had against them. Just finding out the Rohirrim didn't execute their prisoners probably wouldn't have been enough to induce them to fight for their long time enemies. I would think that as soon as they had finished the work the Rohirrim set for them, they had simply gone back home and taken no further part in the War.
As to whether, in time, the Dunlendings would have allied themselves with the West in the Fourth Age, that's possible, but I think it probably would have taken a policy of active efforts of reconcilliation on the parts of Éomer and his successors.