Is your husband supposed to keep cash in his apron?
I'm guessing that the money counted was actually £40 short, rather than correct, but I don't think that'll help you. It may well get the other person disciplined too, but it's not really a defence.
Was he called into the office literally as soon as he got to work? Did he attend work earlier than his shift to give himself time to give the money back and explain?
It'll be fairly easy to make a gross misconduct case from this, so I think I'd prepare for him to lose his job, and then if they give him a second chance it's a bonus.
The poster upthread who said about resigning being better than being fired is incorrect, though, it's almost certain that his reference would say "resigned whilst being investigated for gross misconduct" at this stage, and any future company is likely to take that as an admission of guilt. There's next to no difference between that and being sacked for gross misconduct. Resigning doesn't necessarily stop the disciplinary investigation, either - especially for theft, which could be a criminal offence.
It's really important that you know exactly what is happening here, so I'd recommend taking half an hour free with a solicitor if you can. If not, your husband needs to be completely honest about which rules have been broken and if he had any chance at all to hand the money in before he was called into the office.
I feel for him, I bet he really regrets not calling someone to let them know that he'd found the £40 now. However tired he was, letting a manager know or returning the £40 as soon as he found it may have been his saving grace.