>>>which category of common assault does it seem he would fit into?
There aren't any categories of common assault. (There are different categories of assault, such as common assault, assault causing actual bodily harm and assault causing grievous bodily harm but common assault isn't subdivided - except that where a 'battery ' occurs the actual wording of the charge used is 'assault by beating').
'Common assault' doesn't require any injuries to be caused. You can be found guilty of the offence simply (for example) by pushing someone, grabbing their arm or even by ruffling their hair. If your husband 'applied unlawful force' to the woman in the night club then clearly he's guilty and that is how he should plead.
The circumstances you've described (where both 'harm' and 'culpability' are likely to be classed as 'low' by the court) make it a Category 3 offence, which can only be dealt with by a fine. (Theoretically an offender can get away without even being fined but that's not going to happen). A Category 3 offence can't result in either a Community Order or a custodial sentence. Even if the court was to view the offence as 'Category' 2 (which, from what you've written, would seem to be extremely unlikely), the sentence has to be a fine or a Community Order. (A custodial sentence can't be passed).