Hi, Mensky. Some paternity testing places are better than others. For things to be legitimate, your friend's son needs to use a lab that the family courts in his area of jurisdiction would approve for the legal determination of paternity. At these labs, a person goes with his legal identification, and gets photographed or otherwise identified, and does the test right there with the woman and the child, and witnesses the swabs being taken away by a neutral lab technician and handled in a professional way. If your friend's son and the mother of the child did an at-home test kit, there is all kinds of possibility of fraud, including that someone switched the swabs before mailing. If your friend's son wants to be the father, he should go to a judge and get things arranged for a test sanctioned by the court. A court of law would not use a shady lab or allow the people involved to handle the samples and mail them in themselves.
It is hard to understand your English. At first I thought you were saying a son of a colleague of yours had done a DNA test with a woman and baby, and despite what he thinks to be true, the baby came up as not being his. Then I read it as a colleague of yours (not the colleague's son) has come up as 0.00% chance of being a father of a baby that strongly resembles him. Then I wondered if you were saying you either are the woman in this situation or know her, and she feels the guy should be the father and the test just did not show it.
In any of these scenarios, the problem could be either mistakes or fakery (such as switching the swabs) if the people merely did an at-home mail-in test, or it could be that they went to the cheapest possible lab and the lab wasn't very good.
The solution is to do things with the best available lab, one recommended by the courts. Go to a legitimate lab, in person, with ID, with the man, woman and baby there in person at the same time so everyone witnesses everyone else doing the swab, and do things in a way that a court would direct if trying to determine paternity for legal purposes. If the guy wants another test and the woman doesn't want one, or if the woman wants another test and the guy doesn't want one, the person wanting the test needs to talk to a lawyer and see if doing one can be enforced.