videomike - 10:39 am on Mar 4, 2011 (gmt 0)
In my opinion this is not possible. The only way I could think of would be that the vendor must have access to technology like the alexa bar that can track user behaviour on a statistacal basis. Even this approach would only work if his user base was a regalur visitor to your competitors site in large (like a minimum of 1000 user using it) to gain statistically relevant data.
Why don't you just ask him how he will be doing this because you doubt it is possible. If he comes up with a satisfactory and plausible answer, then you have found something very valuable. Please share your further findings here on the forum :-)