It has been consistently stated that the fight happened 2 weeks or so before Haleigh went missing - other articles state it happened during his visit.
GRACE: Back to Teresa, the grandmother of little Haleigh. What have you heard about an alleged argument regarding a gun? That gun later found in the ditch? TERESA NEVES, GRANDMOTHER OF MISSING 5-YR-OLD, HALEIGH CUMMINGS: There was a gun that was found in the ditch. There was no fight. The gun incident was two weeks before Haleigh`s disappearance.
http://transcripts.cnn.com/TRANSCRIPTS/0902/20/ng.01.html
I watched Mistys mother, Tommy and wife Lindsay give private investigator Williams Staubs a yard tour, showing him a drainage pipe under the driveway where they claim Joe stashed a gun hed allegedly stolen from Ronald; Joe declined to comment.
http://www.artharris.com/2009/10/05/exclusive-inside-the-haleigh-cummings-family-feud/
The Croslins say about two weeks before Haleigh disappeared, Joe and Haleigh's dad, Ronald Cummings, had an issue over a gun. "He stole Ronald's gun. One morning, on the porch, said, Ronald will get what's coming to him. It was dropped, he left and Haleigh was gone," says Lisa Croslin.
http://www.firstcoastnews.com/news/news-article.aspx?storyid=132845
Neves said "Joe" and Ronald Cummings got into a fight over a gun during his visit, and that the cousin left town the same day Haleigh disappeared. But Cummings denies that fight ever happened
http://www.foxnews.com/story/0,2933,498704,00.html
ETA: There is also an article out there that I haven't found again that talks about the fight was words and no physical altercation. I will do some searching of threads and see if I can find the article again if necessary.