This is naturally a relationship that you'll want to preserve, especially if this client is a family member, friend, or connection somehow. Your first customer's experience can make or break you as you go out and see additional customers. That being said, in my personal experience, the fact that your first client has initiated the conversation of coming on as an investor, is a very strong sign for your business. There are a lot of factors to consider when evaluating bringing on an investor (place more emphasis on the type of person he/she is, their marketplace relevance, access to funds, and connections they have that can be beneficial to you). If taking your initial investment from this first customer means you'll have a direct line into pitching your services to 10 more similar potential clients, then that could be a huge win for you.

In summation, there's no reason you should be any more careful in taking on your first client as an investor, than you should be careful in your evaluation of any potential investor. I'd be happy to chat this through with you further and help paint a more clear picture with more details on your relationship.