If someone speaks to someone about him meaning reuvein is speacking about shiman about him (shiman) is it lashon hara even if nobody was there only reuvein and shiman

Hello and blessings, There is a prohibition of Ona'at Devarim (harming/oppressing with words) in this case, and therefore it is forbidden. There are several prohibitions that a person transgresses when speaking Lashon Hara, which the Chafetz Chaim details at the beginning of his book Chafetz Chaim in the count of the negative and positive commandments. In the case you mentioned, one would need to examine each commandment individually to see if it is transgressed, but the act itself is certainly forbidden.