Accidents happen in the heat of the moment but why would you plan to mess up her clothes?
If he's naked, then why would she have clothes on that could get messed up?
If it's a quickie where you aren't naked, then let him cum in HIS underwear & pants. It's his stuff so let his boxers or briefs soak it up. It won't hurt anything and won't be as noticeable as getting it on her outerwear.
As a guy, I think that's definitely the respectful way to do such things.
