No need to fabricate some lie, as that might backfire at some point.

"Hey I would be more comfortable if the vegetables were washed. Do you mind if I wash them?"

Or just offer to help and start washing them.

The important thing is to not make it about them, but about you. Most people don't get offended when you make it all about yourself, and not them doing something wrong.

"Then you're going to have a bad time, because they are on the inside too and your quick rinse only probably gets rid of half on the outside."

As far as I've found, they're both right. You shouldn't have to wash your mushrooms, but it's not a bad idea if you're not buying fancy mushrooms.

The generic button mushroom variants you're probably getting at the grocery store are grown in compost, which often contains some manure - ie poops.

But before growing mushrooms it's pasteurized. Mycelium is picky, and fairly easily out-competed by other stuff, so to make sure you're just growing mushrooms and not bacteria you basically have to sterilize the medium they're grown in.

But those mushrooms are often grown in open beds, and harvested by hand. And that means they get that poop dirt right up on them. Will it immediately give you super botulism? Probably not but it's still kinda ick.

Fancier mushroom varieties from smaller cultivars are the ones that actually don't really need washed and often shouldn't be. They're grown in highly sterile environments and they fruit out of a container, so they never touched the poop. And that's if they even used compost - lots use straw or wood.

If you do decide to wash your button mushrooms it's not a big deal, they aren't actually sponges, and they don't absorb as much water as some cooking shows say. If they get soggy it probably means they're old, try putting them in the fridge for a few hours uncovered. It's basically a dehydrator.

Not only should you wash them, you should also start a sautee with a couple tablespoons of water, then add fats later.

Username checks out

“You don’t wash the mushrooms? It says on the package, man, they might have poop or pesticide or whatever else.”

When we make something about the other person, telling them that they are wrong and, in this case, even disgusting, they are going to get offended and be less amendable to seeing your side and helping you out. They may get shamed into doing it, but I think this damages the relationship.

Just make it about yourself

"Hey, Im a bit squeamish about unwashed veggies. Do you mind if I rinse them for you?"

Gets the same point across, but makes it about yourself rather than the other person, so they have no reason to be defensive. They will also be more likely to open up to changing.

They have a lemon party under the lemon tree.

They wash their vegetables with soap? Like.. special veg soap or just like Dawn or something?

doesn't rinse the soap off her dishes once she's scrubbed them

She European?

Flabbergasted when I learned from an American friend that their British friend did this.

thoroughly wash their hands before doing their business

That would be a first for me to see. Well maybe I’ve seen it at a place that serves hot wings…

I work with my hands so if my hands are covered in grease or dirt or whatever I wash my hands before taking a piss. I do wash my hands after as well.