* You're full time from the beginning, she isn't
* You're contributing actual money to the project
* You're starting the project with committed customers
* Your reputation or background is key to acquiring customers (for instance, you're a doctor selling to hospitals)
* You bring funding connections (for instance, you've founded and sold a VC-backed company)
* The first revision of the product doesn't require advanced programming, so you can get actual customers without a tech cofounder
For some of these, you can grant equity conditionally; for instance, hold back N%, which reverts to the two of you 50/50 on contingency (for instance, specific list of customers don't buy, funding falls through).
Personally, if the word you're thinking of for this person is "partner", you should just do 50-50 with vesting.
I think the big problem with 50-50 isn't fairness so much as that it limits your options down the road. The big problem with 60-40 is that it can wind up being overtly unfair, which can kill your company.