There's part of the issue... no APPARENT reason. Professors are just as entitled to privacy in their personal lives as any other adult. No need to share his personal life with his students.
I'm wondering what you expect a "substitute professor" to teach to your child, if one were even available. (It's not like in public school, where if a teacher calls in sick, a sub is called in to cover the class for the day. And, FWIW, do you expect the public school to hire a teacher every time a faculty member is out? Most subs are parents of students, not trained teachers.) DH spends about 10 hours a weekend preparing for the week's classes, and these are classes he has taught for multiple semesters. He review the lecture notes and power points, making changes depending on student understanding and class progress, adding new bits of info and dropping others, changing in-class assignments, practicing his delivery, etc. There is a certain amount of preparation that a professor puts into a class- intellectual preparation- and lecture notes are really only a brief outline. Professors lecture from experience and knowledge, not notes on a page of paper (or powerpoint presentation- shudder
). Even another professor in the same department can't be expected to walk in, cold, and give a good lecture. Besides, other faculty have their own schedules to keep: Classes to be prepped for, lectures to be given, students to be advised, grants to write, research to conduct, committee and other meetings to attend, grading, parents to be dealt with. People have this odd notion that faculty members keep their own schedules and pretty much float in to teach a class and float out again, doing whatever they want most of the time. It doesn't work like that, but people don't understand this. I'm also pretty sure you wouldn't be happy with a TA teaching. While grad students are smart and hardworking, they also have their own class, study, and research or teaching schedules to keep, so they are (a) not necessarily available and (b) not necessarily knowledgeable in the topic. SO how happy would you be with that?
I think there are several issues here, but the big ones still remain: HOW do you know all about this and WHY isn't your daughter handling this on her own (if it's really even an issue with her)?
It was horrible to overhear that very private phone call. He came back in, finished the lecture, and then took a leave of absence for the rest of the quarter. The dept brought in a TA to finish the class. 
