Lieutenant Commander Jadzia Dax was a friend of Shelby's. During a resupply visit to Deep Space 9 that same year, Captain Shelby returned a favor to Dax by granting Lieutenant Manuele Atoa a day off so he could perform his Samoan firedance at Dax' wedding party. (DS9: "You Are Cordially Invited")
This character was only mentioned in dialogue.
Regarding the reference to "Captain Shelby" in "You Are Cordially Invited", Ronald D. Moore commented:
My intent when I wrote the line was that this was indeed the same Shelby from BOBW. I thought it was a cool reference to throw in for the fans, but I had completely forgotten that John Ordover and Co. had very specifically and very politely asked us if we had any intention of ever using this character again and we (including me) had said, "No way – do what you want with her." This very salient fact was pointed out to me after the show had aired and I had a rather sheepish conversation with Paula Block over in licensing and with John via e-mail explaining what had happened. Since I only used Shelby's last name in the episode, you're free to look at this either way – it's really her or it's someone else with the same last name (it's a big fleet, after all). (AOL chat, 1997)
Peter David later wrote passages into a New Frontier novel explaining that there were two unrelated Shelbys in Starfleet. This was already evident based on the simple fact that this Shelby was a male, whereas the other in question was female.