I think the next question on my mind would be, if this young man is serious, then is he willing to get married in the Church with all that entails, despite his unbelief.
If so, he will likely have to go to a pre-Cana session with daughter that will entail a lot of discussion about God and will go on for one or more days.
I’d also wonder how he is around “Godly things”. Is he generally respectful of your daughter’s faith, or will he be making snide remarks, especially after the initial period of “good behavior” wears off?
There are non-believers who are just fine with all of the above and would be respectful and kind to a believing spouse. There are others who would have certain “rules” (for example, no crucifixes in the house, no discussion of this religious topic or that religious topic) and still others who would be annoyed or disrespectful if they had to listen to too much “religious” stuff.
I am hoping/ presuming that this guy is in the “respectful” category since he has become attached to a young woman who believes. But you/ daughter really need to determine where this guy is going to “draw the line” when it comes to religion, and this can be hard to figure out when like I said he is hoping to marry the girl and may be on his best behavior in order to win her, with his “real self” only coming out later after the marriage.
The one good thing about a long engagement or a long pre-marital relationship is that you/ daughter will eventually see the real person, as no one can keep up a facade for years at a time. By the time I married my non-Catholic husband, I had a really good idea of how he would react to Catholicism, Mass, Catholic stuff around the house, Catholic stuff around his family, etc.