A person who has these tendencies is suffering greatly. He is causing great harm to himself and other beings by acting on the unwholesome roots of greed, anger and delusion.
It does not really matter if you trust him or not. There is still the possibility that he could use or betray you. One cannot control reality. One is just given a hand of cards. It's up to oneself to find out how to play that hand in the most beneficial and wholesome way.
Let's look at both scenarios.
You place your trust in him and he betrays you or uses you.
All he can do is to impinge on your sense-faculties. Anger can arise, sadness or other negative mind states can arise in you. That is really it. Noone can force you to react to these states of mind.
If you instead just observe them with mindfulness in the present moment then you can use these phenomena to cultivate insight. You can use them as a stepping stone to progress in your path.
You place you trust in him and he does not use or betray you.
This scenario is better for him since he does not create unwholesome kamma for himself since he is not doing those unwholesome actions.
For you it's pretty much the same. If happiness arises in you because of that you can enjoy that happiness without being attached to it. As long as you know it is impermanent and unsatisfactory you can enjoy it without dwelling in it. In the same way you can contemplate the conditioned happiness and see the danger in it if one gets attached to it. Conditioned happiness is dukkha in disguise because when it's exhausted it will leave behind a void leaving one wanting for more. That is if one is attached to the happiness. If not you can again use it for the cultivation of insights.
If you contemplate the situation you might find compassion for him. Why? Because he is dealing with the same "illness" that we are all dealing with, i.e. ignorance of the true nature of reality. We are all in the same boat here. Some beings minds are just clouded in a heavier darkness than others. When thinking about these beings naturally a deep compassion can arise.
We are all in this together. As long as we not enlightened we are all dealing with some level of ignorance meaning that we have a mind that is not fully trained to see reality clearly. Only the Buddhas and Arahants are fully awake.
By knowing that we are all in the same boat we can begin to view the situation in a different way.
I do not know if you are training in meditation at the moment but if you do and if you are practicing insight meditation (vipassana meditation) you will come to experience these things for yourself. By seeing this directly through observation of reality compassion can arise in you.
Buddhist ethics is centered on intentions / volitions. When acting according to the Noble Eightfold Path one should cultivate Right Intention (Samma Sankappa), i.e. intentions of renunciation, good will and harmlessness.
It is not your job to fix him or other people. By working on ourselves and purifying ourselves from defilements we can be good examples for other beings. Naturally people want to be around a person that is honest, wise, calm and peaceful.