My personal strategy is to acknowledge this type of person exists, and act accordingly. This means, unfortunately, not putting too much trust into that person in areas they don't deserve it. You need to evaluate carefully both of your interests. Any divergence is a weak point that could be used against you.

I know I can put my faith into that particular person for some specifics such as writing business research and collecting market data, for example. But I could not, for example, leave my computer unlocked with that person. Nor would I use their internet connection to do bank stuff. Nor would I discuss certain things in certain rooms of their house. Some things I'm paranoid for, others not. The point is, some people need to be kept in check ;)