AVHD is a file format from Hyper-V (Microsoft Virtualization). You can't import these files directly because VMware infrastructures uses VMDK as virtual disk file format.
I'm working with a client who sent me a VMware copy of their website
So, this can't be "a VMware copy" because it's an AVHD file. ;-)
I've never worked with Hyper-V, so I'm not an expert in this. But what I read here: Converting AVHD to VHD
is that AHVD is just a snapshot file. And you need a VHD file which you can convert in a VMDK file format.