Is it permitted for a shadchanit to reveal other girls that a prospective boy has already gone out with or is it lashon hora? The information may help to clarify if the boy is suitable for the girl. Thank you.
This is permitted, and it is not lashon hara.
The conditions for permitted such speech are fulfilled in the case of investigations for a shidduch, and since this information can be important there is no problem in revealing who a boy has gone out with before.
Also note that there is nothing inherently “negative” in this matter, and each boy dates different types and kinds of girls. Much is not what you say, but how you say it.