Indeed,that happened me before,the guy left in the car with €35 on the meter was trying to ring yer man cos either way the name on the app has to confirm payment...but neither of them said to me that the payer was exiting first....miles away from the final drop....he eventually ponied up when yer man wasn't answering.
I had a similar instance when a chungwan turned off her phone,
when I went to process payment after she had left it wouldn't
go through, embarrisingly I had to knock on her door asking
her to turn her phone back on fer the 8Euro fare.