Hello brother,
I will offer my own 2¢ as you are seeking wisdom on a tough issue. Tough in that it's not always cut and dry easy to discern.
I've been single all my life, yet an extrovert and been put into leadership positions that puts me in contact with lots of believers and non believers alike. This question has come up many times, so I'll give a few examples and you can perhaps glean something from that concerning your own decision if you wish.
A. Being unequally yoked together refers to oxen plowing a field. They are going the same direction and accomplishing the same goals, day in and out.
When it comes to unbelievers, we may have things in common such as hobbies or coworkers on a job that we see daily. God didn't take us out of the world, but he didn't intend for us to be worldly minded. We are to be lights to them.
An example of unequal yoke would be marriage or a relationship that the other may have inappropriate influence over us.
Example: I thought that I led this beautiful Roman Catholic to Christ. She went along with my leadership for a time, became my girlfriend and we talked about marriage. Eventually, we were growing in different directions. I was growing closer to the Lord and she was acting differently and indulging in inappropriate behavior behind my back. When I realized this, and broke up, she admitted that she was just putting on a show and didn't sincerely trust Christ alone, but put her faith in Sacraments (works, see Titus 3:5) Too. You see, she didn't get saved, but was living a lie. I would not, but IF a believer and unbeliever gets married, or lives together out of wedlock, as is common today, then raising children and many other conflicts will have happened ....for a lifetime. Since I had certain moral foundations, that protected me. Two can not walk together in that close relationship without being in agreement. Some unbelievers do, but then Christ is Not that man's Leader. The head of the man is Christ. The head of his woman is her man, to paraphrase.
C. Non-marriage relationships...
I've had a lot of unbelievers show up to my meetings, but learned that this was not healthy as they may influence weaker believers who need to grow up. Even without intimacy, the primary direction for believers should be God's Word. To unbelievers, it is whatever their own philosophy and morality or lack thereof is.
C. Concerning Reprobates, the doctrine is spoken of many times. Romans chapter 1 is one of many descriptions. I have taught students and run a business where I wasted time, money and energy on them. No longer!
I've learned from too many bad experiences that they don't have a problem. They "have been given over to a reprobate mind.". There are several long lists of their qualities that I don't have time to get into. When I open a business, AND a school, God willing, I will never hire one. I don't even want to take them on as a student. It might cost me money, but Abraham refused the offer of a king's ransom and God still blessed Abraham.
My Dad warned me long before I knew what one was and I have to agree from my own experiences in business dealings. I'm not saying that unbelievers are reprobate children of the devil. All people are children of men. Only a small percentage don't want to retain God in their minds and eventually are rejected by God. Not all atheists are reprobates or agnostic. Some get saved.
I will leave off here with two short messages that might be a blessing. I hope that this helps you. PM anytime.
https://www.kjv1611only.com/video/12other/Jack_Hyles_Explains_Reprobate_Doctrine.mp4
This next 4 minute message from an Evangelical pastor might be helpful for leading your friend to Christ. Let me know if you need help sharing the link with her.
My type of faith is Dying for Christ. That‘s my type of faith. So if someone puts a gun on my head and says Deny Christ or I’ll shoot, i’ll say “Go ahead and shoot“.
