dum == 1
times - lead(times) < 30

mdf <- data.frame(id= c(1,2,2,2,3,3,3,4,4), dum= c(1,0,1,0,0,0,1,1,0), times= c(10,20,20,45,60,70,80,120,170))

id   dum times
  2     1    20

library(dplyr)

mdf1 <- group_by(mdf, id )
%>%
filter(dum == 1 , times - lead(times) < 30)

id   dum times
  (dbl) (dbl) (dbl)
1     2     1    20
2     4     1   120