I have come up with a design/slogan for a line of T-Shirts that a friend and I are working on. When she is done with the art for the design, I will need to approach a printer with this design and have some shirts printed out for me. I have already looked into copyright and all that but that is not my big concern at this point.

What is to stop the printer from maybe liking the design and printing them on their own? One of the printers I have in mind has their own shop where they sell their own shirts...what if they decide they like these shirts and print them for themselves to sell them?

What I am thinking of is when I was programming for a CNC machine, I would often have to sign Non-Disclosure agreements when a client would approach us with a design or product that they wanted us to machine for them. This agreement made it so that I could be sued if I was to use this design myself or give the specs or design to someone else.

Is something like that practical in a situation like this?

Of course. Have him sign a non-disclosure agreement to protect your design and have it notorized.