Customer requests just a 15 min check in?

If you are using the "check in" to send the client. Report card,then yes the Rover app will warn you that you have not been there for 30 mins.
(As long as the booking is not being charged or set up as a repeat drop in) What I have done in the past is not use the report card for that booking. Especially if your not taking to dogs out for walk, its not as important. I will still make sure to write everything down (if not more) information during the visit Letting them know what time you get there, what you do with the pets,if they ate, drank water, went potty, time you leave, ect.) And take plenty of pictures. This way you don't even have to worry about the time and leaving early (per O's request.) I would though to cover your base,is make sure in your message to them you reference the details with the owner stating that she did not want you there for the full 30 mins but instead how ever many minutes.

Just on a side note: I personally would not offer the discount. When owners do request a shorter time,I will let them know the charges covers 30 minutes and give them options or ideas of what you could do for 30 mins. Even if it just hanging out playing with the dogs,brushing them,ect. ( If you think about it, just doing the reports and taking pictures takes time. Not to mention your drive time getting there then back home.) Best of Luck!