My husband owns an accounting firm and he has been approached by a party whose business is online gaming (betting). He has been asked to register a company for them, get them a businesd license and get them a TIN (taxpayer identification number) certificate. Is he permitted to the above for this company knowing they do gaming/betting?
Also, is he permitted to do accounts of such a company once it is registered?

If he is registering a business for this client and this business can be used in any way regardless; then it would be permissible. But if he is registering and licensing it for gambling then this would be considered assisting them in what is unlawful and thus it would be forbidden.
I would avoid such a company like East avoids West, it has damnation written all over it.