My mentor gave me the wrong advice

I have worked with Sam as my mentor for almost three months. We meet monthly for about an hour. He was very useful and had a lot of experience. He gave me some input on my business plan as well as some details about the market and key competitors, which I included before sending it to some investors.

When I met with a friend, she said some of the information I was given by Sam was wrong. I double-checked and found out that she was right.

I got a note back from the investor thanking me for the business plan. They said it was a really interesting idea but that some of financial calculations were incorrect and affect the discounted cash flow. I had done the DCF the way Sam told me to do it.

What do I say to Sam? If I can’t trust him who can I trust?