Define new variable to take on 1 if next row of another variable fulfills condition

so I´m trying to set up my dataset for event-history analysis and for this I need to define a new column. My dataset is of the following form:

ID   Var1
1    10
1    20  
1    30  
1    10
2    4
2    5
2    10
2    5
3    1
3    15
3    20
3    9

And I want to get to the following form:

ID   Var1   Var2
1    10      0
1    20      0
1    30      1
1    10      0
2    4       0
2    5       0
2    10      0
2    5       0
3    1       0
3    15      0
3    20      1
3    9       0

So in words: I want the new variable to indicate, if the value of Var1 (with respect to the group) drops below 50% of the maximum value Var1 reaches for that group. Whether the last value is NA or 0 is not really of importance, although NA would make more sense from a theoretical perspective. I´ve tried using something like

DF$Var2 <- df %>%
  group_by(ID) %>%
  ifelse(df == ave(df$Var1,df$ID, FUN = max), 0,1)

to then lag it by 1, but it returns an error on an unused argument 1 in ifelse.

Thanks for your solutions!