Men can and are the victims of domestic abuse.
If the police had any inkling that he had assaulted her they would’ve arrested him, even if that had been he was released with a caution or whatever. They charged her with assault and the CPS believe they have enough evidence to prosecute.
We’re all going on rumour, conjecture and their own weird social media posts, but what we do know is that SHE is the one who has been charged with a violent crime, not him. Regardless of whether he was cheating, texting, whatever, if the roles were reversed and he’d allegedly hit her over the head with a lamp, there would be no chance anyone would defend it.