Okay, this is my first post to this site, but I felt I might be able to offer some unique help as I am also a 19 year old Catholic girl, and I felt some responses were more intent with being riled up about what they interpret as religious bias than addressing the issue.
Frankly, I am seeing warning signs all over this relationship. The most pressing issue by far is that he is a 29 year old man – 10 years her senior! This might not be such an issue if it was, say, a 30 and 40 year old, but the fact is she, although legally an adult and undoubtedly a capable individual, is still a teenager. She is not biologically developed, which makes it very disturbing that a full adult is going after someone barely of legal age, and seems INCREDIBLY predatory (another warning sign in that regard being that this relationship has been kept from you and has moved so fast). Additionally, she is lacking in life experience which means she needs guidance.
The best solution here, before anything else, is to take time to cool down and then sit down and talk with her as a rational individual, and treat her as such too. Ask her about this relationship – why does she like him? How does he act around her? Why has it moved so fast and been so secret?
Share your concerns in a way that will not make her feel defensive. I have rarely come across interpersonal problems that sincere conversation couldn’t at least help. Be honest, and honestly listen to what she has to say. Tell her you’re worried about how this could turn out, and why it makes you uneasy. Don’t try to force her into anything, as that will close all doors fast.
ASK her to at least wait a little longer, preferably for at least a year or through a university degree. If he is the kind of man I suspect, he will not be happy about having to wait. Assuming she agrees to wait, if the relationship lasts those extra years and you get to know the situation more, I think all parties will be able to make a more informed decision and you can hopefully respectfully intervene. If he doesn’t hang around, which I think is far more likely – frankly, you have your answer to his character and your problem is solved.
I understand she may not agree to wait it out, but I want to stress that attempting to force her won’t lead anywhere good, even if it works. All it will do is show her she can’t trust you with this kind of thing. Hopefully if you explain WHY you want her to wait, and don’t try to end the relationship altogether, she will be able to see the sense in it and agree. If she doesn’t agree, say you want to be more informed about her life in this regard. If she moves ahead with the relationship, she will at least know she can come to you for help and support and you can hopefully help guide her away from future mishaps like this.
My parents have always been very guiding without being controlling to me, and as I result I am very honest and communicative with them. Try to remember that she is likely being manipulated, and that she needs support and not punishment.
There’s a reason your family and friends should meet significant others, and its because you may be blind to warning signs that they will spot!