How can I find how many visitors a site not owned by me has

Sorry did not know where to post this. I have done a site for my cousin and he keeps getting contacted by the following company Copybook . com (Do not want to give them a free back link. ) They want my cousin to advertise on one of their sites - pharmaceutical-int. com/ (again not giving free live link). As per title is there any way of checking them out. They claim the pharma site has 100,000 visitors each week. I am dubious myself. Just as an aside they reckon they usually charge £3k+ for 1 year but they are happy to offer a 1 time special of £1k for the year. My gut tells me this lot are b/shit. My head also tells me this. But I would like to check them out properly if there is any way to do so.
Anyone got any pointers on what I could do to check them out ?

http://www.alexa.com/siteinfo/pharmaceutical-int.com#

That will give you some idea of popularity. Given their top search terms and ranking, there's no way they are getting 100k visitors a week. Also, note that 20% of their visitors bounce, so whatever the true figure is you can discount 20% straight away.

You could try http://sitebeam.net/tests/popularity too, they have a free trial. However, it's based on the Alexa stats provided in the URL above.