I have been asked to give some of our employees the ability to use the Microsoft VPN client (Win2000, PPTP) to...
access a remote customer's VPN server from inside our corporate network. I thought I had read somewhere that this is not a good idea, and at the moment our firewall is setup to not allow establishing a VPN from inside our network. Are there any precautions I should take before allowing this type of connection, or am I worrying about nothing?
Yes, you should be worried. This is not a standard practice in any secure network environment. No access should be granted through the firewall to connect to an outside vendor. Instead, you must create a separate DMZ outside your private/protected space that will allow access to the other company. Use a machine within your DMZ or another DMZ just for this purpose, terminal service into that device and then VPN.
This will protect your company and allow much greater control over the device in case of malicious code, hack attempts or data coming back from the other company.