I understand where you're coming from OP, but your DD is only 26. If she's been with her BF since she was 22, she's probably changed a fair bit since then. What she wants now isn't necessarily what she wanted a few years ago, and as she grows into an independent adult with real life experience, it will inevitably push her closer to her BF or pull them apart.
I understand the feeling. I met my DH when I was 18. In my early 20's, married and with a mortgage, I felt a bit like, "is this it?". I'd look at friends who were out drinking, dating, going on group holidays, whilst DH and I were working and staying in, changing nappies and going to Center Parcs, and I'd feel a bit envious at times. That period has passed as my children have got older and I've I've settled into life. It never made me want to end things with my DH but it did make me feel a bit stifled for a while. It's possible that your DD is feeling the same; dissatisfaction with life, rather than with her BF, and it's something she just needs to work through.
I'd let her talk and just listen. Assure her she can tell you anything, even if it's something she thinks will upset you. Then, if she's not revealed anything untoward about her BF, you might suggest a break rather than an outright breakup. You might try to sus out how she would feel if you offered some support to her BF, too.
Some people would feel betrayed if you supported their ex, but if they are amicable and it's not too painful, you can still reach out to her BF in the early stages.
I really wouldn't try to make her feel like she ought to stay with him if she doesn't want to. If she does do that, she'll end up resenting both of you. You're working on logic - he's nice, kind, loyal etc. - but she's going off emotion and you can't rationalise that.