Ask him how he plans to use it, and what price he/she/it is willing to pay for it..
Having been down that road once, I can assure you that occultism is about seeking personal power for personal purposes, something that is always strived for but never actually realized without paying a much greater price than it is worth.
Make sure that that person knows that when all else fails to bring fulfillment, Christ is still patiently waiting for him and will be there if called on.
The problem with asking about the price (as one who has also walked that road) is that the price is far from apparent. The universal forces are there, free for the taking, and all you have to do is tap into them (at least that's how it looks at first). There is no price, or the price is based on the concept of "a gift demands a gift" where something is offered in exchange for what is requested. From the outside people who have been there can see that the nominal sacrifice is nothing to do with it, it's simply a mask to cover up the true cost, but until you see clearly you won't see the true cost. And it's hard to see clearly when you're caught up in something that prevents you from seeing clearly.
Not everyone seeks personal power for personal gain, I've known a few people who got at least lightly into occult practises with a desire to help others. These people typically wanted to do things like healing. The price for that is even less apparent.
It's good to remind them that Christ is always there, but don't be surprised if it takes a long time and prayers that will frequently feel like they are totally ignored.