Keffah says she watched as they would then take the man to a tap and let him drink -- but they would continuously slap him, beat him up and curse him while he drank the water. She would then hear the old man thanking them for giving him water.
"I would sit crawled up in a corner in my cell and cry so much thinking about what they were doing to this man," she said.
As she told the story whispering so no one could overhear in a Damascus café, Keffah was overcome with emotion.
"One of the worst forms of torture is the psychological one," she said as she wiped away her tears.
On the day of her release, Keffah recalls being taken down into a car. In the back was a blindfolded man with his hands bound.
As they were led out of the car and into the elevator, the guards removed the blindfold off the old and frail man.
Tears rolled down his cheeks as he realized his fellow detainee was a young woman.
Keffah sat in the room where an investigator met with them before their release.
The old man thanked the investigator.
"When he spoke I realized this was the same old man who would beg for water. I remembered his voice," she said.
As the investigator gave the man his belongings, he apologized: "Sorry old man, we apologize, we picked you up by mistake."
Keffah said crying: "All this! All he went through! And it was a mistake?! I will die and never forget his face."
Keffah and Zaidoun were among the tens of thousands of Syrians who took to the streets in 2011 in what began as peaceful anti-government protests.
But after a violent regime crackdown, some demonstrators began to take up arms.
Others defy government shelling and sniper bullets to provide humanitarian aid to those in need.
Keffah is an Alawite, the same sect as President Bashar al-Assad, but she refuses to be identified as that. "I am a Syrian," she says.
She denies that her country is being engulfed by a sectarian war. She says her group delivers aid and reach out to the different communities in an effort to keep Syrian society united against a sectarian rift.
Both Zaidoun and Keffah are opposed to any international military intervention in the conflict. They believe a negotiated end to violence is still possible and want a political process that would turn Syria into a democratic state.
But as the brutal civil war that has claimed nearly 70,000 lives continues with no end in sight , they say the plight of the tens of thousands of detainees is forgotten.
When asked how many people they know who are still detained, Keffah says: "It's a long list. A hundred faces are flashing through my head at this moment."
People are detained at random. Keffah and Rami were detained as they arrived to meet with a contact. Zaidoun was detained at a Damascus café after he showed up for a meeting.
Families of those detained spend weeks and months looking for their missing loved ones, not knowing if they are dead or alive. In some cases they get messages from released detainees.
Keffah says she spent her 18 days in jail trying to memorize the names of the dozens of detainees she said would be brought in every day.
In the facility where Zaidoun was held, he says the ages of detainees ranged from 13 to 75.
Torture is rampant and many they say are forced to confess crimes they did not commit.