First, congratulations! This sounds like an exciting opportunity. However, part ownership/partnership can be tricky. What happens if you disagree down the road? What happens if someone gets sick, dies, or has to leave the company for any reason? What happens if there is a divorce and a spouse who isn't currently involved in the business also becomes a part owner?
Have you considered structuring your compensation to be salary plus a share of the profits without true ownership? That could potentially reduce headaches and liability for both of you.

Congrats. There's a lot of variables. What is your role? How much are you being paid? How much equity are you given? How many people are on the team? Is their a board of advisors? Is there any investors or potential investors? What phase is the startup in?

Feel free to connect with me and send a call and we can go through it.

I've been in the startup space for awhile and I can help you sift through by asking the right questions and helping you to make an educated business decision.

Marketing is not what marketing used to be. In the last 4 years marketing has shifted from Agency, demographics, target marketing and block and tackle product management/marketing to a Digital Monster. To survive and then maybe succeed, you have to understand how 75% of marketing has shifted to technical automation, behavior modelling, and the systems that track and help control the behavior, and then the monetization engines required for today's marketing. It is not just a website anymore or analytics.

First of all, a big congrats. All u need to know at the first sight is the potentiality of start-up he is wishing to start. have a thorough research, and considering part ownership, is good as far as my view is concerned, because risk is directly proportional to reward. For any of your help, i am just a call away, anytime.