Do you mean the Wifi signal disconnects, or just that once he VPNs in he cannot connect to anything else on his Wifi network?
for the former:
Nothing about VPN should cause his Wifi signal to drop unless he's using a custom Wifi stack with some weird compatibility. (e.g., he's NOT using whatever isjust built into Windows or Mac OS X)
for the later:
This is a very common thing, and is quite possibly intentional. Basically, when you VPN somewhere, if you can connect over the VPN *and* to local systems on your wifi or local ethernet, this is called "Split Tunneling". For various reasons I won't get into, this is considered a security risk to the organization you are VPNing to, and is often also a policy violation for various compliance requirements they may be subject to. So the remote end will often forcibly disable your ability to connect to local systems while the VPN connection is active. (i.e., "Split tunnel is not allowed").
If the remote end is not intentionally disabling split tunneling, it is probably a minor configuration end on your error. If you have a Mac, it's simple to fix. Open Sys Prefs, Network, and reconfigure the list of network adapters so the VPN adapter is *under* the Wifi or Ethernet. If you're using Windows or Linux, not so simple. (It's simple if you're familiar with the 'route' command from the command line. but if you were, you probably wouldn't be posting this question

)