You should talk to a lawyer in your jurisdiction and perhaps read some books so you understand the general lay of the land. The level of generality at which you’re asking this question makes it difficult to give a concrete answer, and if you’re worried about liability and how to paper your arrangements with investors, you should be speaking to a lawyer (consider looking for an entrepreneurship law clinic if you can’t afford to pay an attorney at this stage).

You should ask for at least enough equity that you feel compensates you adequately for the time you are putting into the project, such that you don’t either move on to something else because you’re getting underpaid or feel resentful down the line because you were underpaid relative to your time investment.

One of the things that lawyers and others in professional services have to learn quickly is that you’re not betraying a personal relationship by asking to be adequately compensated when it leads to work for you. It’s the same here - you’re selling your services to her, she’s buying your time and taking you away from other lucrative opportunities, and you should ask for the market rate for your services. You aren’t doing her a favor here - she’s buying you at your market rate, as converted into the estimated value of the shares.

As a litigator who is currently litigating a case on behalf on behalf of a CTO cofounder who was screwed over by the CEO cofounder with an MBA: make sure you are getting separate advice on your rights and not just relying on the CEO/company counsel to understand your rights. “Business divorce” destroys promising startups; get separate representation at critical points early on and it will save a lot of expense and risk to the business later on.

This isn’t to say you should distrust your cofounder, but understanding exactly where you stand prevents fallings-out down the line.