Advisor or Co-founder?

Hi Elphas! Here is my situation, my startup has a scientific/psychological component. My co-founder and I are not experts in that space, leading us to find advisors in organizational and clinical psychology. We engaged an amazing Postdoc in clinical psychology who is working with some companies. She is amazing, and we like her a lot. Currently, she is helping us to craft an important piece of our product. Before starting working together, we asked her about her time commitment, and she mentioned she could work part-time because she is creating her consultancy firm. Based on that information and a few documents we found in Y-combinator, Founders Institute, and Carta, her role description fitted as an advisor. So, we offered her 1.5% - 2%. She commented that a friend in the startup world told her it was very little for her work. I agree that her contribution is an essential piece of our startup, and I want to be fair. She is not a one-in-a-month call advisor, but she is not a full-time founder. Also, since this is a Saas, we will need her the most at this early stage to automate the process, but I am unsure if her psychology services will be needed in the next stages with the same "intensity" as right now. Definitively she is very valuable to the team, and we don't want to lose her. What do you think? have you been in similar situations? How much should we offer? Does she serve as an advisor? (Note: we're bootstrapping, and we can't afford to pay for her services also, she wants equity, and we want to have her onboard)