He knew he must search or look for a sympathetic person. He had to search to find this person. When he met Dr. Bruyn, he found one (the sympathetic person). One in this case is a pronoun which refers back to the previous "sympathetic person". "Found (one) in" means that the sympathetic person that he needed was "found in" the character or personality of Dr. Bruhn".

He found the sympathetic person he was searching for ... it was Dr Bruyn. This is a phrasal verb