The reason the London Embassy want all 'criminals' to apply for a visa is that they don't trust the UK public to have the foggiest notion of what moral turpitude is - and to judge by many posts in the past here and elsewhere they are probably right to do so. They therefore wish to decide the moral turpitude question for themselves, and the mechanism for doing so is applying for a visa.
You are perfectly entitled in US law to decide that question yourself. The only slight (and yes, very slight) danger in doing so is that IF the Immigration Officer knew of the conviction (and yes that's a very big IF) and disagreed about it not being moral turpitude then you've no way of challenging him.
Your friend's chances of getting a visa if applied for depend on how long it was since his crime (10-15 years might be OK), and how serious it actually was. They are not improved by his having lied to get a visa waiver.