This question already has an answer here:
Is this answer from here stating that an external observer can see a black hole mass grow, correct?
This question (is an external observer can ever see the mass of a black hole increase--motivated by the standard view that it will take an infinite time for the external observer to see the mass crossing the event horizon) is a question that comes time and again in [different variations] but asking basically the same. Many are marked as duplicates and when you go there that one is also marked as a duplicate in an endless loop (well, it ends somewhere). I also see this question is other physics forums. The multiple answers that really try to address the point are always similar: No, we cannot see it grow, or we "see" the mass growing but is actually outside the horizon (but at the same time we can never measure that), and many other variations. Many of the answers actually contradict a little each other. I do not know the actual identity of any of those who answer, and of the few that I know their identity; they are not really experts in the field.
Then I found the only answer different to all of them. It is here and it says that "he new matter to cross a critical surface that is outside the original event horizon of the smaller black hole. This occurs in finite time, even from the external observer's viewpoint, and the black hole simply grows in size" and " The time is infinite in the limit when the mass of the infalling object goes to zero and there's nothing else that makes the black hole grow. So the infalling object is just a "probe".