No, it won't harm the car, or your dad. As soon as he starts the car, the exhaust will blow the balloon off the tailpipe. If you clamp it to the tailpipe, it'll blow up and pop. If he has the windows up, he probably won't hear it above the radio.
A word of caution here: If you clamp the balloon to the tailpipe and your dad drives off, the rubber of the balloon will melt to the tailpipe as soon as the car reaches its operating temperature. If you dad has chrome tailpipe extensions, you could be buying him a new one. At the very least, you'll be trying to clean the melted remains off the tailpipe.
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Naaah, it will just blow up and then pop when it gets too big.