If she has had a boyfriend, she's got to be in high school, right? Or at least in middle school?
So why have you not empowered her to speak up for herself and to refuse to walk with the obnoxious boy? Why have you not urged her to ask her friend why he associates with a kid who does this? Why have you not told her to stop walking with them?
50 degrees is irrelevant. People don't get sick from being wet. But that's not the point - this would be wrong if it were 80 degrees. She needs to stay at school until the problem boy heads home on his own, and if asked, she can tell the staff why it's not possible to walk with him. Her ex-boyfriend can decide which of them he values more.
If you go to the boy's parents with a complaint or a video, you won't be bolstering your daughter's own ability to stand up for herself. She needs to develop this skill. There's never been a better time for women and girls to stand up and say, "I'm not going to take this anymore." Point out plenty of great role models in the news right now. She has rights, and she has the right to not be touched or assaulted, even with water.