1. This is normal practice for a law firm. In order that you get advice from a law firm, a letter of engagement is standard, particularly where you are engaging the law firm on a joint basis with another person whose interests and yours might not always be coincident. Additionally, if you want advice from a law firm, you will have to pay for it. No law firm advises a person without getting paid for it. Essentially, the law firm are simply formalising the relationship at the outset, so no issues subsequently arise as to whether you were a client or not and whether there was some conflict of interest with their existing client who they are already advising. Finally, you should realise that just because you have seen a copy of a report from a law firm, this gives rise to no liability on the part of the law firm for the contents of that report and how it affects you. The position is very different once you are a client of the firm because then the law firm has a duty to you to give you correct advice.
But this is rather my point. I would not expect a law firm to produce such a report without a letter of engagement.
2. But you are not a client and the report does not pertain to you until you become a client by signing a letter of engagement. So you cannot rely upon it when deciding whether the enter into a business arrangement with this individual. You would be very foolish to do so.