It’s best to be subtle when contacting DNA matches that are the result of an unexpected parentage. Lying is never advised, but it’s good to remember that the person you are contacting may have a different reaction than you to finding out that a near relative had a child that no one knew about.
Even if you think you know the probable connection, it might be best to initially indicate that you are trying to determine the relationship more precisely than the results page indicates. And there is still no guarantee that you will ever get a response. If the individual’s tree is public, you might even want to take screen shots or save their information just in case they decide to make the tree private or take it down entirely. One never knows just how someone will respond–particularly when the unexpected relative is a fairly close.
And if you are not certain that your analysis of the results is correct–ask someone who is knowledgeable about genealogical DNA test results to look at the information. And…when you do that don’t tell them what your conclusions are. They should be able to put those aside when doing their analysis, but that way you can really see if they reach the same conclusion that you did.