I see no issue with this, in that they are no longer a student/tutor and are not young and vulnerable in this sense.
However, if they were still at university, this would not be ok. The tutor is in a position of power, similar to a boss. If the relationship had started/looked likely, then the tutor would need to (at my work anyway) essentially resign as their tutor and pass them on to someone else, not mark their work and so forth. Similar to how a boss shouldn't be doing appraisals of someone they are shagging.
It might all be lovely and consensual and the start of a great relationship. I have heaps of colleagues who met in similar ways. However, I also have heaps of female colleagues who have been patronized, harassed and pressured by their male colleagues, often a lot older and in a position of power in their workplace. There are quite a few sexual harassment suits flying around in my field at present for these type of activities.
So- as it stands, I think it is fine but if they were still student/tutor, not the same at all.
Also, and this is just a caution, many younger women end up entranced by older cleverer men in academia. Unfortunately they are a few years behind in their careers and they imagine they will move forward together, both benefitting from working together. Often-times the woman then has children and falls even further behind. By this time, the man is much more senior, perhaps a Professor and has no intention of picking up the slack at home to jeopardize their enjoyable conference attendance around the world.
Not always, but I know lots of academic women who just haven't realized their potential, and their husband really haven't helped them either. They are then stuck in part-time/fixed contract work or just leave.
So- my main criteria would be- will this man help your dd to be her true self and fulfill her potential, or is he looking for a personal assistant to help him become Professor whilst she..doesn't? It can be hard to tell, but she should watch for this over the next year or two and assess whether she has truly met her equal.