When you say "final event ever," do you mean final event within the date range you are filtering for, or the date range across the entire data set?
In the former case, there are a couple of options I can think of, I set them up in the attached using the Superstore Sales data, using Customer and Order Date in place of userID and lastEventTime. Option 1 filters on the Order Date, and has a second filter based on MAX(Order Date) that is set to show only relevant values. It would be ideal, I think, if the second filter were able to select for individual days, but Tableau does not give us that option for continuous date filters (and the MAX(Order Date) field needs to be set as continuous for Tableau to allow it to be a filter, since Tableau only allows continuous aggregated values to be used for filters).
Option 2 uses the same filter on Order Date as a starting place, then a parameter-based filter to choose a date.
If you are trying to get the final date across the entire data set, you can use the parameter-based filter to flag this, I also set that up in the attached.