What part of the process are you doing? As for the question at hand, the real answer is that it is up to you, and your friend. I would suggest that if you do go forward with a partnership set up, that you seek legal council to make sure that this is set up the right way, and to ensure verbal agreements can't be "forgotten" or "changed'' over time.
If you go into a partnership, you need to clearly define responsibilities and duties, to each other and the company, and what will happen if either of the party wishes to dissolve the agreement, and go their own way. What will happen with the company, and what is the leaving party owed or what do they owe?
Generally the advice around this forum is to try to avoid partnerships. If you search the word "partnership" on the forum, you will find many threads with much advice and opinion on them. They do happen and there are ways to set them up so they work out, and so that you are protected from the foreseeable things that can go wrong, and the unforeseen. That is where your legal counsel will payoff. Find a lawyer that specializes in partnerships/agreements.
If you can, maybe you can "hire" or contract out to your friend. This way if things change, you can outsource to someone else. I always feel the person designing the clothes is needed more than the person printing the clothes, so you have more to loss (if you are the designer.) He can print anyone's clothes, but only you can create your unique designs. Make sure you can continue on in the future profitable designing your clothes, with or without a printers help. You can find other quality printer, alot are right here on Tshirtforum. If you feel like it, you can get some quotes from printers here in the recommendations and referrals section, as well, folks can recommend good printers in your area if they know of some.
If your designs really take off, do you really want to split your profits 50/50% with your printer/distributor. I am sure Johnny Cupcakes does not do that. You may not become as big as Johnny Cupcakes, but when he was starting out, he also didn't know he was going to become so big either.
This post isn't to knock any printers, far from it, but it is written with Tony in mind, and Tony's best interests in mind. Quality printers should be paid fairly for sure, but should a printer be a partner? That is the question here. Ultimately these are random thoughts, Tony, for you to consider. Business can be done in many ways successfully, how you set yours up will rest solely with what business model you see as being the best route for you. Best wishes to you.