I understand you don’t want to lose your daughter and her willingness to confide in you. That is perfectly understandable and yes, that is the ultimate goal.
But.
He is not decent. 55 and 21? Is he older than her father, your husband? It’s creepy regardless because he is actually even old enough to be her grandfather. Can you not see why your dh finds this predator disgusting?
The incredibly large age gap is very much a factor in the power and influence he wields having been her teacher. That is why a relationship with someone her own age who perhaps had once been in a position of power would be a concern but not always wrong.
In this case in all sorts of ways it is creepy, unethical, and unprofessional.
However, you do have the reality to deal with: what can you do about it without harming your relationship with your daughter.
Keeping in mind there is going to be risk no matter what you do including and possibly especially if you do nothing, this is what I would do.
I’d have my husband (yes given the serious situation I would take charge of this and give orders) call the appropriate person, not email or letter, call and explain that professor who is 55 is soliciting a romantic and probably sexual relationship with a former student, your 21 year old daughter. Your husband should do it because he is the one who feels strongly about the ethics of this.
I bet the professor will drop your daughter posthaste.