My friend has found out that he has some Jewish family history according to DNA tests.
Could it help him to get Israeli citizenship, or is this fact not considered at all?
It's not considered at all.
There is some DNA that is indicative of Ashkenazi Jewish heritage, but it's possible to have it and not be Jewish, and it's possible to not have it and be Jewish. (i.e. it's a clue, but it's not definitive in either direction.)
It's a marker of ancestry, not of religious status.
Now your friend can check his history and see if perhaps the DNA indicates that he actually is Jewish. This would require that his mother, and her mother, etc, etc all be Jewish.
But if he got this DNA from his father, then he would not be Jewish.
Also, it's possible get Israeli citizenship even without being Jewish. The process is longer (Jews are essentially considered as if they are all already citizens), but it is possible to go through it - it's similar to the process needed for naturalization in any ordinary country.
As Ariel said, it is not considered at all. However, there is more considered than what Ariel wrote.
The Israeli Law of Return is designed to provide sanctuary to anyone who would have been persecuted under the Nuremberg Laws of Nazi Germany. The exception within the Law of Return, mostly in order to preserve the Jewish character of Israel, is that someone who was born Jewish but deliberately changed his or her religion (ie., not someone whose birth mother was Jewish but who was raised by adopted parents to be Christian), is not eligible, even though that would not have saved someone from persecution under the Nuremberg Laws.
But, as stated above, a DNA test is of little use in this regard. Any person whose mother, father, grandmother, or grandfather (on either side) is or was Jewish (if they are deceased, but without having converted out), is generally considered eligible for Law of Return status in Israel.