How can i get my friend to stop liking my ex?

i have this friend named jackson and he likes this girl. the only thing is... that girl is my ex girlfriend. and he stalks her all the time. and shes got a good sense of humor so she made up this joke that 6+2=9 and my friend said it was 8 but when i told him she thought that it was 9 he "rethought" and he makes her freaked out and i want to SABOTAGE HIS DATING LIFE!!!!!!!!!!!!!!!!