I think that if your friend is working on the business and contributing in areas such as product design, positioning and market validation (and I daresay in usability with real customer archetypes he finds), and also in the preparation of the business plan, deck, and sourcing of angels, seed financing sources and the like, he is contributing as a co-founder.
If he's sitting back saying "atta boy" and "I'm with you when the product is done" or "I'm willing to spend time on this when you can pay me" then he is not a co-founder.
He is a "friend of the company" and potentially an early hire.
If he wants to invest $75K in the company, write him a convertible note and take his money. However, this does not "buy" him a job or an operating role in the company.
He is a chicken.
You are the pig.
Think bacon and eggs.
The chicken is involved.
The pig is committed.
You own this company.
When you find a co-founder willing to dedicate blood sweat and tears to this venture, no matter what amount, you must return that commitment by extending him a slice of equity.
The amount of equity you award is based on:
1) The stage of the venture
2) How essential are his contributions and what are the nature of the contributions
3) How much you need him at this time in the company's path or stage
4) What you design into your stock option model as "equitable" for a given role
In this you might look at a way to say "VPs get this much stock" "Directors get that much stock" "Senior Engineers get this much stock" and so forth.
What you need early in the company is a structure or model such as this, in order to avoid pain, chaos and hard feelings associated with uneven or perceived inequities in stock distribution.
I suggest (fair disclosure: I am on the board) a system such as Option Sanity. It is free or nearly free for small operations, and is a great way to get clarity on who gets stock and why. http://www.optionsanity.com/
You have not met the right person to co-found this company. Anyone can have good business judgement and relevant experience. Not everyone is suited, by their DNA or by temperament to be a start-up founder or co-founder. Writing a check is the role of an investor, and still limits their role to that of a chicken, to a great extent. If they need to keep their day job, this is understandable. They can still commit and do great work and make great contributions in the 20-40 "other" hours of the week that they don't give their employer. You need to meet someone who can be on-fire in their belief and commitment to the company and its goals and mission.