I'm going to help my brothers ex best friend
What is making you decide to help your brother's ex-best friend?
I want to help because he's just a cool person.
If your brother and his friend parted ways on bad terms, then you might consider how your brother would feel about you helping his friend. He might feel you are disloyal to him, and he may be hurt. You should consider who is more important to you; your brother or his former friend.
My brother and I are adults, so he has to let me help the guy out if I want to.
That is understandable, if you like the guy enough to remain his friend then that is your prerogative and your brother will most likely understand. Just make sure to respect your brother and let him know what you are doing, so that things aren't awkward between the two of you.