Can not ping internal systems while on VPN
Contact the people who maintain the Fortinet VPN. They probably disallowed split tunneling, which can cut off local LAN access by forcing all traffic through the tunnel. (Presuming that clients A and B are on a private network). Changing your windows firewall won't do anything about this; this restriction won't be anywhere that you're allowed to make changes in.
The VPN administrator will typically be following a policy that says to configure the VPN this way, but if you can make a good pitch, you might be able to get yours needs accommodated.