Can I be sued for not giving someone promised equity?
Yes, you can. Especially if it’s writing, even an informal email.
It happens all the time. You can even see a variant of this (I know, not the same thing, but a variant) in the Winkelvii-Zuck drama at the heart of The Social Network.
The risk with a prospective hire is probably close to zero (not zero, but close). But the risk if you shake hands with a co-founder is real.
This the #1 reason to put the longest possible vesting schedule for all co-founders. With a 1+ year cliff. This cures a lot of related issues re: anyone that isn’t there in 12 months.