I've seen so many relationships "fail" when there's the usual two or three year gap.
There's no guarantee on the longevity of a friendship or a relationship of any kind.
As far as age is concerned, it may help if the maturity of both, matches up better than their birth ages.
How would you feel if you "had a word" with your daughter, then she took your advice, and she ended up with a person of much closer age.... THEN .... she became so unhappy because she didn't feel as close, or as stimulated, or worse, he might turn out to be one of the abusive twats that unfortunately you read about on this forum.
Everyone's life is their own to either get it right, or make mistakes.
I can so understand the fears you have for your daughter, as my own dc had a relationship that was so subtly toxic, it wasn't apparent to them, as they were "so in love"....
....................I went through mental anguish whilst that relationship lasted, but I kept my opinions to myself, was pleasant and even friendly to the person....
......................................time went on and eventually my dc saw for themselves, without any interference from me.
Of course it was still devastating, but at least it broke before marriage was planned.
X amount of years later, my dc is married, and there is an "age gap", but this time my dc has thrived in this relationship, they are loved and supported, and that is also the case for the partner.
As much as it pains you, I doubt your daughter would want your disapproval, no matter how well intentioned.
All any good and decent parent can do is to "be there" for their child when something goes wrong.
Just as you would want to share in your daughters joys, you may have to also share her woes without the umbrella of "I told you so" or "I thought as much".....
Good luck OP, being a parent never stops being potentially difficult, no matter what the age of your children.