Have you not talked to your co-founder yet? That seems like an important thing to do before you make your decision. I understand that you may think he is so invested in the company that he will see you considering the offer as a betrayal, but tough decisions like this are exactly the kind of thing you should talk over with your co-founder and best friend. If you accept the offer without talking it over with him, he will very likely see it as a betrayal. Perhaps he will see an upside to having a best friend who is a VC? Give him a chance to make his own reactions. No matter what road you choose to take, you will be happier in the end if you keep your best friend.