I guess that you can just explain to your client that there is no need to bother at all with the "robots.txt issue" however hard it is to rank for his keywords, you can always rely on your SEO of the webpage plus the optimized links that you are going to build for his website.

There might be a difference in the time frame to accomplish such task but if you only continue to do things right, nothing can stop you from taking his website to the top even anywhere you want it to rank high with. I've had a client from NZ in a really competitive keyword in real estate and it took me just a month to take his site to the top page at # 6 and his site is not even hosted in NZ.

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