What to do about fellow student that hogs lecture time with many questions
Unless this student is a close friend of yours, I think you should bring it up with the course instructors, not directly with him. The person who is in charge of making sure the course is operating smoothly and well is the instructor. In a class with more than a few students, spending 25-30 minutes responding solely to one student sounds like a clear mistake to me. (The fact that this is happening in multiple classes and none of the instructors have done anything about it is curious to me, but I don't really know what to make of it.)
I would make an appointment to speak to each instructor in question about it. I would come to that appointment with specific information about other students who feel the same way. If we are talking about a few students, then probably you should come to the office together. I am a university professor, and if several students came jointly to my office to express concern about the way my class was run, I would have to take that very seriously.
If the number of students who are concerned about this is more than could comfortably fit in your instructor's office, then I think you should make a list of such students in some way, e.g. by writing up a brief, politely worded statement describing your feelings on the situation and getting other students to sign it. Or perhaps even in the second situation it might be better to start with a few students who come visit your instructor's office. Things to worry about in the second situation are (i) you don't want to attribute a complaint to a classmate unless you have specifically talked to them about it and gotten their approval and (ii) if you don't handle things with enough discretion it could get back to your inquisitive peer in a way that you don't want.
Perhaps others will disagree, but I really do feel that if the behavior is acceptable to the course instructors then it has to be grudgingly acceptable to you: I would not try to persuade your peer to change his ways. If the additional time spent on your inquisitive peer has consequences in terms of the learning experience, you should bring those consequences to the attention of the instructor, specifically and repeatedly if need be. For instance, if the lectures do not cover all the material that the syllabus says they should, you could ask the instructor what to do about that and whether the exams will be adjusted accordingly. (This is a pointed, but fair, question.) If at the end of the course it turns out that indeed the learning experience was compromised in this way, then you should give appropriate feedback about it.
The only case I can think of in which I would confront your peer directly is if you have as much discussion with the instructor(s) as you can, they agree that the questions are derailing the lectures but are themselves unwilling to do anything in response. They are then really not doing their job, and I think at that point you would be justified in trying to intervene: it seems like the least evil.
I wouldn't make an appointment. Simply wait until the next time this student uses up more than 10 minutes in a class. At the end of class, approach the instructor and wait until others leave. Then say "Joe's question really cut into our class time today, and not for the first time." This should start the conversation in which you can express your concerns about material not getting covered, or you not getting to ask questions, or it just being boring to listen to Joe's one-on-one tutoring sessions.
Why do I not want you to make an appointment, bring other students, bring a list of who else feels this way and won't fit in the office, and explain to your prof specifically what gets missed when 20 out of 50 minutes are spent tutoring one student? Because it's patronizing. It's making a big formal deal out of something that your prof already knows. Some "help vampires" are very good at derailing intelligent people, who intellectually know they should not be helping the person right here and now, and getting their help right here and now.
Given that your prof knows intellectually this shouldn't be happening, all they really need is a little reminder and support, as close to the incident as possible. Waiting for an appointment, gathering your posse, meeting with others to write out the (incredibly obvious really) consequences of the time spent inappropriately in lectures - this may all feel exciting and important, but there's no need for it.
If your class has a post class tutorial, Joe should be dominating that instead. I had a student who would use the whole hour if no-one else came. If someone else came, he would let them ask all they wanted and use all the remaining time. We had some very interesting conversations. But he never interrupted lectures for them. If you don't have a tutorial session then Joe should be going to office hours or whatever is needed to grasp the material. Your prof already knows this, but is having trouble saying it when Joe's hand goes up. Don't try to teach your prof this, just remind and support, that's all.