if the mineral spirits he is using are mainly denatured alcohol should not damage the paint in any way. Since the overspray is fresh and not fully cured, he will likely soak a rag in the spirits and wipe down the car, this will remove the overspray quite easily, just make sure he does a thorough wash and wax afterwards as all your existing wax will also be stripped off.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)