Legal actions will not be applicable since the daughter is an adult. But whatever the age of their children, parents should talk with their children and give wise and practical advises. It's their duty to do that, whether they accept or not. It is important for parents to make efforts to communicate with their children from childhood and develop bond of friendship and trust.
Can a parent take action if their 19 year old daughter is having a relationship with 59 year old man?
What would a 59 year old want with someone 40 years younger than him? He's using her for arm candy. And seriously WHY would a 19 year old want to be with a 59 year old? They have nothing in common and BOTH need to think that over. Having said that, at 18 she was an adult and the parents can do nothing but let her make this mistake.
Unfortunately no. Although it's rather stupid, what does she think is going to happen as he ages.
When one hits the age of 18 they are considered an adult and can date any one of any age. Unless, the person is disabled then they may be some thing you can do if the person is mentally at an age younger than 18 and the 59 year old is taking advantage of the 19 year old who is disabled mentally. Even then that is very hard to do and prove. Most likely there is nothing you can do.
You should have a way to tell your daughter he's old and you don't approve. Let her know you love her and to look for signs that he's too old.