According to the rules on the US embassy website, any arrest automatically bars a person from using the Visa Waiver Program:
http://london.usembas..._and_convictions.html
However the actual wording on the ESTA aplication page is slightly different:
"Have you ever been arrested or convicted for an offense or crime involving moral turpitude or a violation related to a controlled substance; or have been arrested or convicted for two or more offenses for which the aggregate sentence to confinement was five years or more; or have been a controlled substance trafficker; or are you seeking entry to engage in criminal or immoral activities?"
https://esta.cbp.dhs.gov/
Since your husband's (alleged) offence does not qualify as 'moral turpitude', he can truthfully answer 'No' to that question.
Further, the US immigration service have no direct access to the criminal records (or arrest records) of any other country except Canada. EU privacy laws bar Spain, and the UK, from opening up such records to outside agencies. Only specific information (e.g. relating to possible terrorists, drug smugglers, etc) can be passed on, on an individual 'need to know' basis.
I can see no reason why your husband would find any difficulty entering the USA under the Visa Waiver Program.
Chris