Try http://www.spyfu.com/.
To be honest, if my client told me that, I would tell them that would be a complete waste of time, unless he/she was trying to win a pissing contest against their competitor. Instead of trying to find an already existing keyword list, generate a targeted keyword list of the highest revenue generating keywords for your clients product/service. The odds are that the search terms his/her competitor are targeting only have a handful of searches per month with very little revenue.
It's not hard to find the keywords that bring in the highest amount of revenue for products. Simply use Google's keyword tool, find the related keywords for the product/service your client is selling, and sort them by the highest CPC. I normally don't even add kewyords to my keyword lists unless the average CPC is $0.15-0.50 per click. Any keywords with average bids below that tells me that no one is making money on those keywords and hence are not worth the time going after.
Of course, I would be able to give better advice if I knew what type of product your client was offering. For example, a online product, a local service, a digital product, etc. all have different strategies.