The embassy has stated, as I said earlier, granting a visa does NOT guarantee entry. A border official can refuse you. Once they see the offences you have owned up to they are likely to refuse you IRRESPECTIVE of owning up.
I think you've misunderstood: being granted ANY kind of visa, with or without criminal convictions involved, does not guarantee entry to the USA. Neither does having a green card, although you have more legal rights, including the right to be seen by an immigration judge.
The only people who are 100% guaranteed entry are USA citizens.
If your friend applies for a B visa (tourist visa) and declares the convictions, he may be initially denied the visa and have to apply for a waiver of inadmissability to get it. Or he may be granted the visa, after an interview at the embassy, at which he should stress how much his life has changed since the historic convictions, demonstrate clear regret for his actions, and generally give a sterling impression.
If the London embassy staff grant him the B visa, it will show US immigration staff at the point of entry (airport) that his convictions have been properly assessed and he's been deemed to show good character and be no present threat. YES - he may still be denied entry, as might anyone who tries to visit the US. He may have to go to secondary for extra questions, given the serious (in US immigration terms) nature of his convictions. But the London embassy staff won't just casually issue him a visa, assuming the airport immigration officials will properly deal with the issue. He'll have been thoroughly screened, and immigration officials will bear this in mind when they decide whether to actually let him enter. It's by no means a foregone conclusion, or even especially likely, that someone who has declared all convictions and been granted a valid B visa will be denied entry.
This isn't an 'oh, they'll never ever let him in even if he does everything right' situation. I echo a previous poster's advice: check out the US section of the British Expats forum. There's no point getting into an emotive debate about judginess and whether a person can change: none of that is relevant to the actual legalities of US immigration.
He can do what he wants, but it's illegal to misrepresent yourself on an immigration document, he does have other options, and there are potentially serious consequences to consider.