1. Ask the customer for the jar and read the contents. This will give a clue of what you will be dealing with (sugar, dyes etc)
2. As the customer what they have put on it. Again this will help to decide on what can and can't be undone
As this is a thick substance if untouched you should have a sporting chance.
I always start with the mildest working my way up the scale so I'd go with very warm water and then go with various spotters. At the end of the scale is basically bleaching the area if you are confident enough with met sul. If you get that far then I'd get a disclaimer signed allowing you to do whatever you like with no comeback.
As a very last resort, if the carpet newish then you could cut/paist a new piece in (taken from somewhere else)