She was unsure, uncertain if she could actually handle being a sub. Things could easily go too far should she submit herself to a man and only then discover he wanted more submission than she was really willing to give. So he thought of a way to offer her a chance for safe submission. It would require trust on both sides, but what D/S scene doesn't? A public meeting, with instructions given, commands to go and carry out away from him, with an expected return to him with some small proof that she had actually carried out the commands. Since he couldn't be with her to monitor her compliance, he would have to trust her in some regards. And she would have to trust him to not give her commands that would expose her to inordinate risk. And commit to complying and honestly reporting back her sucess or lack thereof

A parking lot, what looks to be a random chance meeting. Apparent casual conversation. In reality, a handover of a small "chip clip" style clamp. And an instruction to into the ladies room of the nearest store, Target, maybe a Lowes. Apply that clip, carefully chosen to not too strong, to a nipple in a stall. Keep it there while masturbating until sopping wet. But no orgasm. Return to the Dom, who has passed the time on his phone in the parking lot.. Report that there was not an orgasm. Present perhaps damp panties to demonstrate that she did indeed get as excited as required.