Did Harley Quinn kill Robin, was she just an accomplice, or was she uninvolved completely?
Harley Quinn first appeared in Batman: The Animated Series in September 1992.
Whereas Jason Todd was 'killed' in "A Death in the Family" which is a four-issue Batman comic book story arc first published by DC Comics in Batman #426-429 from 1988 - 1989
Since Harley didn't exist at the time Jason was killed, she couldn't have been involved.
There's an onscreen graphic that Harley was an accomplice to the murder of Robin in Suicide Squad.
However, since there is no "real" name for the Robin in question, we don't know which Robin this refers too.