Long story short: she was planning to distract him (by renewing their sexual relationship) in order for him to be blamed and thus take the fall for the corners she cut trying to close the selling of the company to Conally & White or whatever they're called.
Why she chose to do so by a sexual encounter is because:

• They got prior history, so it should be relatively easy to revew something they once had
• She's a serial harasser, so it's easy for her to use that particular tool
• She figured that even if he refused she could always pull the sexual harassment card on him and that alone would suffice to distract him (why she didn't mind at all being exposed at the mediation, that was simply to stall him).
  The book makes a better job at explaining all this.

It almost is a guilty pleasure to experience. Imagine you had that one shot at her and never took advantage of the opportunity, you'd be regretting it for the rest of your life.
He had no reason to regret it because they had once hooked up and even lived together years before. Their relationship had included sodomy and all other things. Having sex with her again wouldn't have been very tempting, which is why he was finally able to resist her.

Was she actually trying to hook up with him to get the job, or was she going to frame him for sexual harassment?
Both.
They wanted him gone.
They wanted cheap parts, little overhead and the $100 mil from the merger. She was going to get a one night stand and claim sexual harassment to get him out of the division. They would use the portable CD-ROM to make the merger look good even though the particular production line they were using was screwing them up. As she mentioned, they were only going to use them for a year and then move on to something else.
It was a cash grab not unlike what start-ups do today by getting a "great", "innovative" product Kickstarted, picked up by investors for millions (or billions) and then it fades into obscurity shortly thereafter.
Ouya, anyone?