Nolan Wells' friends say he hung back on Horn Island with a girl when they left in their boat ... but the girl is apparently telling a different story.

19-year-old Katie McCormack spent time with Nolan on the island before his disappearance and death, according to the Daily Mail ... and Nolan's friend, Warren Hudson, says their group was frantically rushing off the island as their vessel took on water ... claiming Nolan refused to leave Katie and join them on their journey back to the Mississippi mainland.

Thing is ... Katie's sister, Gracie, told the Daily Mail ... "Katie had a conversation and assumed that Nolan was getting ready to get on the boat he came out on."

Katie's sister added ... "Katie assumed the 'friends' telling him to get on the boat wouldn't have left him."

As you know, 18-year-old Nolan never made it home from Horn Island ... and his body was found July 6 in its shallow waters ... near where he was last seen alive.

Investigators are still trying to piece together what led up to Nolan's death ... and his family commissioned an independent autopsy.

Preliminary findings revealed he may have suffered a head injury ... but there's no official cause or manner of death yet.