Is it inappropriate for a student to attend their mentor's dissertation defense? I'm an undergraduate student at a research one university. My former instructor and current research mentor is defending their dissertation soon. I asked if I could attend and they said they would be fine with it. 

My primary interest in doing so is to see how the defense process works for when I attend graduate school. I am also just genuinely interested in supporting them since we both research the same material and they've been incredibly encouraging and helpful to me. 

I know defenses are open to the public, but, in your opinion, would a student attending your defense add more stress or disrupt the process? I just want to make sure I'm not making anything more difficult for them or overstepping a boundary, even though they say they are totally okay with me attending.
It's fine. As you say, these are open to the public, and it's common for family members, department members, and friends to attend. Since you perhaps are not "firmly" in any of these categories, asking whether it's okay to attend is probably a good idea -- but you've already done this and been given the green light. Enjoy.