Best Evidence.
I trust that you are looking out for the spiritual well-being of your friends, and not just to comfort them.
Your friends have had quite a hard time so far in life, and no doubt the time ahead of them will be difficult for them too and for you also. They'll need your support if they are to get through this coming period of time. Pray for them earnestly.
You seem relatively certain that your friends are saved, and that is good, but keep in mind the possibility they are not. It would be a terrible tragedy for them to be deceived into living in a false hope of Salvation.
Sadly, in many Christian circles today, there is the tendancy to think that as long as we say sorry to God, we can live pretty much how we like and that's ok. But that is not the case. We are warned in 1 John 2:3-6
"3 And hereby we do know that we know him, if we keep his commandments.
4 He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
5 But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.
6 He that saith he abideth in him ought himself also so to walk, even as he walked."
John here was talking about those who know the truth, who know what is right, but willfully give in to, and live in a particular sin out of the lust of their own flesh.
We are given a list of such sins in Galatians 5:19-21
"19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God."
Your first step, whether your friends are saved or not, is to show them that they are sinning against God. It is important that you don't just say that they are, but that you show them the relevant texts, showing them also God's warnings. Encourage them to only consider what God's says and not to consider other peoples opinions, or even what other people think of them (they may be experiencing some dissaproval from others - this will put pressure on them to do the right thing, but it is pressure of the wrong type, because they'll do it for the wrong reasons).
They should not being sorry just because of any earthly consequences, be that in friendships, or status as Esau, but sorry because they have sinned against God and offended Him, I cannot emphasise this enough.
We read in Heb 12:16-17
"16 Lest there be any fornicator, or profane person, as Esau, who for one morsel of meat sold his birthright.
17 For ye know how that afterward, when he would have inherited the blessing, he was rejected: for he found no place of repentance, though he sought it carefully with tears."
When we read Genesis 25 through to the end of 27 and beyond, we find that Esau was only sorry because he had lost out, he depised Jacob and was bitter against him, but he was never sorry because he had offended God. This is why Esau found no place of repentance. His heart was focused upon the things of this world and not God. In order to find a place of repentance, we first must have a broken and a contrite heart realising we have sinned against God, and secondly we must turn away from our realised sin.
True repentance does not mean just saying sorry, but it also means turning away from that sin. Saying sorry to God but willingly continuing to live in sin is not repentance. It is a terrible offence to God, because that sin is being idolised and put higher on that persons agenda than God.
We must mean what we say. There is a hymn that says:
I often say my prayers,
But do I ever pray?
And do the wishes of my heart
Go with the words I say?
It is the heart that God listens to, not the words that come out of our mouths. Jeremiah 17:9-10 says
"9 The heart is deceitful above all things, and desperately wicked: who can know it?
10 I the LORD search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings."
God searches heart, and rewards us according to our ways and our doings. Someone who repents will not only say they are sorry, but they will show they are sorry. It is this repentance that God listens to, not our seeming concern, or the words that come out of our mouths.
Once you have pointed your friends to scripture passages that show them without doubt that they are sinning against God. Point them to the remedy, Christ, through verses such as
John 6:37 "All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out."
Matthew 11:28 "Come unto me, all ye that labour and are heavy laden, and I will give you rest."
When you have carefully done this, give them a few days and pray for them earnestly. If your friends are saved, they will seek not to live in sin (encourage them in this also). In practical terms, this means that they will either seek marriage counselling and get married (but live apart until the time they do), or they will in spite of their own natural desire, live apart. Until they are married any sexual relationship they may be having must come to an end, because it is fornication. They'll struggle with all this and will need every bit of help they can get.
If they are not saved, then that sin will be too strong for them to break, and they will be unwilling to part with eachother. Even in this situation though, all is not lost, you have a wonderful opportunity to bring the Gospel, the message of salvation to these dear friends that you have.
It is not the words of a prayer that God hears, but the heart. If your friends want God to hear them, they must truly repent, and not just say sorry.