Do you want this individual as a co-founder?, because that is essentially the role they would be stepping in as. At minimum you would need to write a solid vesting plan of some kind, to prevent the individual from quitting in a month and taking the equity with them.

That said, I would be somewhat suspicious of the situation where someone is willing to work for only equity. Are they already independently wealthy? Are they having trouble finding paying employment?

What do they bring to the table and how badly do you need them?