Dudley grew up and got married and had kids. Harry visits somtimes and talks to Dudley while the kids play. Its been known that Harry's kids dont like Dudleys. It's unknown about what happened to Harry's aunt and uncle.
We know that all three Dursleys went with Order of Phoenix members Hestia Jones and Dedalus Diggle into hiding from Voldemort. So if Dudley survived, Vernon and Petunia certainly must have. In which case, it is quite probable that they returned to 4 Privet Drive in Little Whinging after the war was over.