But Reeve and Brown's shock and horror over their diagnosis was soon replaced with a mission to warn Padieu's other girlfriends. One of their first calls was to a woman ABC is calling Megan, a neighbor of Padieu's who had dated him for two years. She also tested positive.
The three women decided they needed help and contacted the Frisco Police Department to see if something else could be done. Cpl. Tom Presley took the initial call and complaint.
"It was clear to me that any intimate contact between Philippe Padieu and his victims was consensual," Presley told "20/20." "The difficult part that we had was to prove whether or not he knew that he had HIV when he had unprotected sex with them."
But while the police started to build their case, the three women decided to start their own investigation.
"We wanted to protect other women," Reeve told "20/20." "We didn't want it to happen to a single, solitary other individual."
The women say they had good reason to be worried about other victims. Megan, who still lived near Padieu, had seen women coming and going from his home. So, the women say, they took down the license plate numbers of the vehicles and had a friend track down the owners through the Department of Motor Vehicles. They called at least 23 women, some of whom were more receptive to the news about their boyfriend, Padieu, than others.
The women then took turns watching his house, followed women out of Padieu's driveway, even went through his garbage looking for possible names and numbers.
"I got one lady to pull over on the side of the road," Megan said. "She said I'm moving in with him,' and I said, 'you know, I'm HIV positive and I'm quite sure he is, too.' And she then moved her things out."
The police department, the prosecutor's office and the local health department knew what the amateur sleuths were up to and were very supportive.
"They were a remarkable group of women," said Presley. "They were feeding me with lots of information to make my life easier and to make the case successful. They were the ones that did the work."
While the police were trying to get more women to come forward, the county health department showed up at Padieu's house with an official document ordering him under Texas health code to "cease and desist any activity which puts others at risk of infection."
Padieu was warned about infecting others. But that did not seem to faze him, Reeve and Brown said.
"He was court-ordered," said Reeve, "and had a car in his driveway that night."
"And the night after," Brown chimed in.
But these desperate housewives were hot on his trail, and Padieu was beginning to feel the heat.
"He didn't know where it was coming from but he knew something was up," said Reeve. "He knew he was getting busted."
Watch the full story on "20/20" Sunday, 9 p.m. ET on "The Sixth Sense."