You didn't mention how they brought the book to you - specifically, did they actually ask you for any feedback? Unsolicited feedback is usually unwanted. If they show no indication of actually wanting critical feedback, then they probably do not want it and are not expecting it from you.
If they were actually wanting the kind of feedback you want to give, they would say something that acknowledged your expertise and specifically requested help, and acknowledged that you might feel uncomfortable giving criticism, like: "We know you have an academic background. What do you think we could do to improve this book? Please be brutally honest". I'm guessing from the question they did not say this.
If all they said was, "Hey, we wrote this book!", that's a request for shared social celebration, not feedback. So while you believe their project would benefit from editing, they didn't actually ask for your opinion about it, and it's not your responsibility to give people help they didn't ask for and don't show the self-awareness that they need (as in, they didn't say, "Here's our draft, it needs a lot of editing, but what do you think of the ideas"--this would show that self-awareness).
If they said something with ambiguous scope like, 'What do you think?' tread carefully, as they could just not know any other way to ask for celebration, but still not actually want critical feedback.
In summary, give them your best guess about what they consciously want in sharing it with you, rather than your idea of what the project "needs".
Lastly, if their audience for the book is not academic, the grammar mistakes may not actually be a problem. If their audience is people with their same level of education and from the same culture, it might actually be fine. So you might ask them what their plans are for the book before determining that it can't succeed as it is.