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Assuming he still needs to do this. L-carnitine is probably the fastest legal (ex-liposuction) way to burn the excess fat in that time. You can buy it as a powder.
It will bind to the fat stored in your body and transport this to be used as fuel (instead of what you just ate/waiting for it to be produced naturally). Not sure I would recommend it's use long-term, but it works.
1 stone in a month seems like a lot though
Calorie deficit. 3 pounds a week is easily doable
That's ~12,000 calories a week deficit assuming an average man needs ~17,500 a week (7 x 2,500) to break even; you can only consume ~5,500 calories all week to lose 3 pounds = less than 800 calories per day.
Possible? Yes. Easy? Hell no!