Also I was wondering how do we (and scholars) actually know that the Gospel of Mark was written around 50-60 AD and that Luke and Acts was written in the 60’s?
It is debatable whether Daniel is a prophet in the Deuteronomy 18 sense because prophets are people who speak to God’s people, saying “thus says the Lord.” Daniel was not a prophet in the usual sense. You should recommend my book on Daniel “Daniel, Prophet to the Nations” (available at www.ipibooks.com ) to your friend. It will answer many of his questions. I talk there about the fact that Daniel is not a prophet in the standard sense that Jews view a prophet. And even if he is a prophet, he is a prophet to the Gentiles not to the Jews.
About Daniel 9:24, every aspect of this prophecy has been fulfilled completely, but we must put it in its proper context. The phrase “finish transgression” can also be translated as “restrain transgression.” When Jesus came he brought salvation. Those who are baptized into Christ are baptized into his death and for them the sinful nature is put to death. They do not completely stop sinning, but sin is restrained, and it is completely forgiven. For a Christian, sin is put to death. This does not mean that sin ends for everyone, but only for those who are in Christ. Sin was also atoned for on the cross. With the ministry of Christ, prophecy was brought to an end. Hebrews 1:1 tells us that in the past God spoke to us through many means, including prophecy, but in the last days he speaks to us through Christ. Hebrews 2:4 tells us why, soon after Jesus and his immediate successors, prophecy did in fact cease. And of course, the Most Holy was anointed. Jesus is the most holy one, the anointed (the Hebrew word is mesioch or messiah). So, all of this prophecy was fulfilled in Jesus four hundred and ninety years after the decree to rebuild Jerusalem in 457 BC, in the seventh year of Artaxerxes (see Ezra 7). If we take the New Testament at face value, then all of the elements of the prophecy in Daniel 9:24 are fulfilled in Christ.
Your friend says that it is an “unverifiable claim” that Jesus fulfilled the prophecy about the atoning for wickedness. I agree that we cannot scientifically prove that the death of Jesus provided for atonement of sin, but the biblical story certainly implies that. So, if the skeptic does not believe that Jesus fulfilled this prophecy, I can understand that, but to a Christian he certainly did fulfill all of these things, although I will admit that there is an element of faith on this. The claims in Daniel 9:24 involve things that cannot be measured or quantified. Science does not help with this, but we can say that Jesus came to Jerusalem at the proper time, and that Christians believe he fulfilled all the aspects of this prophecy and that belief is rational, even if it cannot be proved scientifically or mathematically.
We do NOT know for a fact that Mark was written between 50 and 60 AD. In fact, it is possible it was written in the forties. Nearly all scholars believe that Mark was written before both Luke and Matthew, but this is based on reasonable arguments about who relied on whom. It is not provable. There is very good evidence that Luke was written while Paul was still alive, but after he was brought to Rome. This was probably in AD 64 (or possibly AD 63 or 65) It is a bit harder to date Matthew, but it puts the destruction of Jerusalem (in AD 70) in the future, not the past. So…. It is a reasonable conclusion that Mark was written before about AD 63, and fairly likely it was written in the 50s AD, but there is literally no way to prove this. All we can do is ask what is the most likely date. Luke is easy to date. Mark and Matthew are harder, but the vast majority will put Mark before Luke. We cannot know the exact year of writing of these books. All we can say is what is the most likely conclusion.
Liberal scholars and Bible skeptics have an agenda to put the date of writing of these books as late as possible so as to justify some of their beliefs. So, you should look at their conclusions with some skepticism. Of course, skeptics will argue in reverse about conservative Christians, so you will have to decide for yourself, as will your friend.