If he makes a one-way commitment without making them commit to anything in return, he makes himself look super desperate, he removes the urgency to lock him in as a candidate, he makes himself look desperate, he incentivizes them to delay the final contract for as long as they can, and he frees them to explore other potential candidates in the meantime. At the same time, by signing such a promise, that neutralizes his ability to negotiate, so it allows them to potentially reduce the salary and reduce the benefits they were originally willing to give him. This is in addition to the exploding offer nonsense, which reduces his ability to receive competing offers. In short, it's an all-around very bad idea for the candidate.
And one last thing, you said a "headhunter"? Do you mean to say that the person is a 3rd party recruiter? If he's negotiating with a 3rd party recruiter, he's negotiating with the wrong party. He should contact the company directly. By negotiating through a 3rd party, any promise/concession made by the headhunter can easily be reneged on because the employer can just claim that the headhunter was not authorized to make such a promise/concession in the first place. But of course, the reverse won't be true, you can be assured that anyall the promises/concessions made by the candidate makes to that 3rd party will be remembered and noted down and could potentially be held against him by the new employer (even potentially in a court of law). So it's not only a waste of time negotiating with a 3rd party headhunter, but it can only hurt the candidate to do so as well.
