Who is the old man that finds out where Andrew Cunanan is hiding?

TV Guide explains:

Did Andrew fire a warning shot for the man who came looking inside the houseboat?

Yes. The man in charge of watching over the houseboat — which by the way belonged to a man who ran gay bathhouses in Las Vegas and is believed to be a place Andrew knew he could hide — alerted authorities when he went to check on it and noticed the front door was compromised. "His suspicions mounted as he and his wife entered, because all the lights were on and the drapes, which were always open, were drawn," Vulgar Favors says. Cushions had been pulled off the sofa and made into a bed on the floor with a blanket, and a chair had been turned over as if to form a barricade. As he pulled out his gun, he heard a loud shot from the second-floor master bedroom. He and his wife ran outside and hid in the bushes and he called his son, who called the police.

Vulgar Favors is the book the series is based on:

Though the Versace family contends FX's series is fictitious, the series is based on the book Vulgar Favors by investigative journalist Maureen Orth — a Vanity Fair correspondent who spoke to more than 400 people in researching her exhaustively detailed book.