For your specific example, I would actually take dad's side (even though the basis is 100% off). There's no harm in telling your mom to take a few classes, that she's smart and capable enough to get back in school.
I understand this take, but the issue here isn't HOW I respond to my mom but that I should be able to CHOOSE how I respond to her and what my relationship with her looks like. If she complained about the education gap to me, I might (or might not) respond as you suggest without prompting (she hasn't; I live 500 miles away intentionally and we don't talk often).
The boundary that needs set is not with my mom, but with my dad, to make it clear that I do not want to be his proxy in his efforts to always meet my mom's emotional needs. He has always placated her, despite the harm it caused us as children. Now he is getting older, and frankly I think it's harder to keep up with her because he has less energy and age-related health issues. More and more, he's "tagging in" the kids for help of this kind. And to clarify, the request isn't to simply encourage her that she could take a few classes; it's to research options that he can share with her, and, as my sister paraphrased it, to "pour it on a bit heavy" with compliments about how much we appreciate her.
One thing I've discussed a lot in therapy is how I struggle to express genuine affection with my family members, despite the fact that I do love them deeply. I do not have this same challenge with others who are close to me. One reason for this is because I feel like everything in my family is performative, as the whole family unit has adapted to keep my mom on track. She needs grand gestures and flowery expressions of love and devotion to fill the void of her own lacking self image, and my dad is conditioned to provide that. And she assumes that everyone else needs that same sort of over validation, so she has also conditioned my dad to interact with everyone in that way. If he fails to provide enough flattery or profuse gratitude to someone when she think it's merited, she will lecture him.
So now he interacts with me in that way too. We planned a 50th anniversary celebration for them this summer, and I think he thanked me about 5,236 times. After the 1,000th time, I ran out of ways to say "you're welcome." As someone who values authenticity and simple but sincere expressions of affection and gratitude, I find this play-acting utterly exhausting. But if I opt out, I'm labeled ungrateful. I truly want to have close relationships with my family and be able to express to them how I feel, but if that's ever going to happen, it has to be in authentic way. These kinds of requests make authenticity feel impossible. I know it sounds like a small ask--just compliment her. But it's a never ending cycle and it honestly feels soul sucking.
The "there's no harm in {insert seemingly benign request here}" approach is actually the mindset I am trying to sort out how to address as I navigate this. Because, in fact, there is great harm. It's once again putting my mom's needs in front of everyone else's for the sake of peace. Watching that example growing up is what landed me in an abusive and toxic marriage. How to explain the harm in something that seems so benign on the surface is where I struggle.