I have a scenario I am trying to configure where I need to restrict a customer to only 1 IP address inside our network. This customer will be connecting via VPN and I am not sure how to do this.

Have you tried this:

http://wiki.mikrotik.com/wiki/Manual:Interface/L2TP

Like it shows there, you can specify remote-address on /ppp secret, so that client always get the same IP.

I have a scenario I am trying to configure where I need to restrict a customer to only 1 IP address inside our network. This customer will be connecting via VPN and I am not sure how to do this.

The task can be achieved with variety of ways. Among many, One is described below ...

When the remote user will connect to your Mikrotik VPN Server, he will get local IP address, make this ip static in that user profile, so that he should get same ip every time.

Now Create a FILTER rule, that should block all requests from this IP address and in destination, type your required server ip which you want to allow him and use EXCLAMATION mark, means EXCEPT this destination ip, block all other. This way all request [EXCEPT the REQUIRED SERVER IP], will be denied from his ip