Seems kind of late-stage to be bringing on a "co-founder".

For any new employee (and you don't need to get hung up on the titles), they should generally get an equity grant that is proportional to the value you intend for them to bring to the business.

If you can't grow the business to the next level without this person, they get a stock grant similar to yours. If they are going to implement some "really important stuff" but generally not be involved in the business management they'll get somewhere between "quite a bit" and "a pretty good chunk".

Generally speaking, it's not a good idea to allocate 100% of the stock. You should think up front about what other employees you'll need to hire, what you might have to allocate to investors and so forth and make grants accordingly now.