More details would help...is this an employee? A client? A prospect?

Overall I'd say leave them alone to stew in their own juice, if they're tied up with a negative mentality. You cannot change people. When they're ready to change, they'll find you again.

If it's really important, and you're willing to risk the relationship (however thin it may be), sit down with them in person if possible. If not, live by phone. Not by email. And confront them about it.

Sounds like:

"John, I really want to help you here. I hear you saying you want help. Do you really want help to get out of your situation?"

(wait for answer)

If positive: "Okay. Can I be kind of blunt with you?"

(wait for answer)

If positive: "All right. Promise you won't get mad?"

(wait for answer)

If positive: *sigh* "I've noticed something. You're free to agree or disagree of course...here it is. Every time I've tried to talk with you about this and get a solution in the works, you've blocked the conversation with super-high negativity. It's stopped me cold. I get the feeling you're really not interested in fixing this: that what you really want is someone to complain to. What do you think?"

And pause.

You're going to get a response here, one way or the other. Either the person is going to erupt defensively, or they'll sheepishly agree with you. In either case you now know for certain what you're dealing with.

If positive: "I really do want to help you. Do you believe I can help, if we talk through this, come up with a plan of action, and you go carry it out? Do you believe you can change the situation?"

Speak in a nurturing tone throughout. Never argue. What you're doing is uncovering the true motivation here. Either the person wants to change or they don't. You cannot make them change, or want to change. Just discover their real motive.

If negative anywhere along the line, it's probably time to leave the conversation. You can do a takeaway, "All right, I guess this just isn't for you" and get up to leave, but it will probably remain that way.

With prospects, it is best to "fight" up front rather than later on after money has changed hands. You want to know exactly who you're dealing with before you take them on. If this is a friend, you may want to go the "Leave it alone for now" route.

Keep in mind at all times that you cannot change people; they can only change themselves. You can't want it badly enough for them that they'll make the change.