Likewise, I wouldn't deduct money - you don't know if it was done through carelessness or was an unavoidable accident or had nothing to do with your helper (eg, leaky iron or problem with washing machine).
Tell your helper your husband is very cross about the shirts (not you - you're the good guy!) and suggest your helper could try to find out the cause and just be really careful when doing the laundry. Stress that you don't know what happened and suggest your helper try to find out what the problem is. Give her some responsibility.
At the end of the day, we are lucky to have helpers - we don't pay them the wages of household staff in London or New York. They haven't been to 'butler school' or 'housekeeping school' or 'nanny school'. They are simply human and we all make mistakes. If a couple of ruined shirts are your biggest problem, I think you're pretty lucky. Yes, it's annoying - but it's nothing in the grand scheme of things.
Remember - don't sweat the small stuff!!!
;-)