Domain mapping from 2 servers

Hi

I will try to explain as better I can.

I have a WP multi site instalation and I have a client server,
the website from my client is something like client.com/shop and I like to know if is possible to create a start page on my WP multi site page to show like client.com and when press to go to shop go to client server on client.com/shop.

Both nameservers are the same but I don't know if this can work or not.

It is possible to do this?
How difficult is to do this?