You should inform your client of the situation then work together to resolve the problem. Best course of action might be to bring in another expert you know tem

You should inform your client of the situation then work together to resolve the problem. Best course of action might be to bring in another expert you know temporarily, possibly through subcontract if your client don't know who to hire. Calling in another expert is not an admission of incompetence. It's just a matter of efficiency and expediency. I'd not recommend doing what lawyers do. Best client-developer relationship is built-on trust. Follow your heart and you'll be rewarded over time.