According to The Real Barenziah, Book II, Barenziah had slept with him to gain entry to Thieves' Guild. He didn't only tell her about the skills she needed to become a member of the Thieves Guild, but also trained her the skills himself or else introduced her to people who could. Among them was a woman called Katisha.
One night, Therris summoned Barenziah and Straw to loot an estate, which turned out to be General Symmachus' estate. Straw was set outside as a lookout, Therris and Barenziah sneaked in. However when two figures entered the room and caught the thieves, Therris betrayed Barenziah and ran, but he was caught. The captor turned out to be Symmachus himself.
As a punishment for betrayal Barenziah suggested twenty stripes; she did not want to do him any permanent injury, just teach him a lesson. However, Symmachus had Therris drawn and quartered.