I haven't even read all the posts yet. I was wondering, when I read how mom called you (really? At his age) and was saying son was crushed about this whole thing if she wasn't being over-protective because her son has a behavior type issue (one track mind/doesn't listen/gets a thought in his head and won't deviate from it), whether it's a disability or not. She certainly isn't doing him any favors coddling him.
I think both the kids messed up, the boy more than DD. First, no means no. Second, I can see a confused "okay?" to mean, "yes." Third, I can't imagine how they (he) got from the "okay" to a "just in case you change your mind" question to renting a limo, restaurant reservations, corsage, without so much as a conversation to verify that she would in fact be going with him.
DD will need to be more assertive in her "no's" in the future. This is a learning process. She should also pay attention to the personality types. She knows this kid gets onto a subject and stays there, and knowing that, she will have to be bluntly clear that the answer is no, especially with people like the boy. She will have to hurt feelings and step on toes once in awhile. This is life. This is a skill she will learn. Rather than "okay?" she should have said, "I have plans already. I can't go."
This boy needs to learn how to stop and pay attention and recognize social cues and also not take everything so literally. "Let's get together for coffee" or "maybe we could ride together" is not necessarily a commitment. Touch base and solidify plans instead of sitting at the coffee shop or showing up at the house. "You said we'd get together for coffee." "Er, yeah, but not this weekend, dude." His mother will need to work on that (with him, not with other friends/parents), and he'll learn it himself after he gets let down a couple times. You need to follow through.
This whole thing is a little nutters and this mom sounds like a piece of work. I wonder of the boy was mortified his mom called the other mom. Ack! Your daughter would be wise to stay away from this mess.
Also, I don't understand this thing around the boy having to ask the parents before they ask the daughter on a date. Is this how you raised her, that's fine, but a lot of boys won't know this, so it will be up to your daughter to explain it to these boys when they ask her out that they need to call her father first. I know as a teen, I had to ask permission from my parents and my parents had to meet him. None of this "honk at the curb" shenanigans or "meet you there." I wouldn't get to go if they didn't get to meet him, but there was no requirement that the boy had to call my parents before asking me out, that part was my responsibility.