Try this: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PbYTAA0
If that doesn't work and all he has is the URL, then there's not going to be an easy way to do it (that I know of).
What might help a bit is to go into the Design Studio and click on "Landing Pages" (don't drill down, just click on the name of the section). This will display a list of all landing pages and also what campaign they belong to. It won't show the URL, but it's a high-level view that might be a better place to start.