Csdrhrt

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.

I think @cag51's answer ("it's fine") is actually a bit too mild. Please go if you can!

Attitudes towards thesis defenses seem to vary a bit from program to program. My PhD program had a pretty strong (but informal) expectation that people would attend defenses, especially if they a) worked together b) worked in the same sub-field, or c) were friendly. That expectation is weaker in my current department: students do go, but it's acceptable to skip defenses due to non-specific 'busyness' too.

On a more personal note, the final stages of grad school are often lonely and miserable, or at least it was for me. It was therefore incredibly touching to have people turn up for the defense: the room was packed with friends, family, collaborators, and even a few new people who were inexplicably interested in my work. To the extent that your presence triggers an emotion, I think it's much more likely to be gratitude and pride ("Awww, she came!") than stress or confusion ("What's she doing here?!").

Plus, there might be some free food afterward!

In the similar situations I know (habilitation kind of dissertations) it will be seen bizarre if you are not there, both on scientific as well as human relationship point of views.

This is a normal thing to do. I was in a similar position and will be attending my mentor's dissertation in May. It's nice to give back to someone who has helped you out a lot by supporting them during a very challenging experience. Just make sure he's okay with it. He will probably be very encouraged!

It's fine, I have walked into stressful defence-type lectures which one of my teachers was teaching. I don't mean to put the person off, but it's standard and you have to get used to it. When I first started teaching even though I was nervous I invited my friend to sit in on one of my lectures as it's something you have to get used to. Anyone can walk in there, you could be giving a stressful lecture one day and a family member you haven't seen for ages will randomly walk in and sit down to watch.