Selling chevy truck then mystery water in oil

[Just puttering]

My friend decided to sell his 03 full size Chevy truck. He posted it to sell and a man and his son came to look at it. The man wanted it and test drove it with his son. They were gone about 10 to 15 minutes and showed back up and said they liked it, opened the hood pulled out the dip stick and showed my friend the oil was frothy.
They said oh no it has water in the oil. We still are interested but will you take less for it. My friend said I cant sell you a broken truck and they left. Later they called back and asked if he would take 2000 less for it.
He decided to sell it to them over needing to put money into it.

So .... the question is

Have you heard of a scam like this? I think they messed with it to look like it had water in the oil.

Do you know of engine cleaner or something that would look like water in the oil?