Most likely the issue is with timeouts between the user's RDP session and the target computer. This can be caused by general traffic on the network his laptop is on or the office network. The WiFi doesn't help either. The Citrix connection will remain active because it is not timing out with the server. I get this problem all the time working from home.
Is there any reason your user can't use a traditional VPN and have all the applications needed installed on his laptop? It is generally much faster over RDP.