Not documenting is a terrible sin, especially for someone who is being paid a great deal of money and who is not going to hang around to perform the needed maintenance. However, divining what the code does and writing the documentation will certainly be a worthwhile learning experience. If you can find the original requirements documents and specifications (assuming they exist!) they should make the job easier.

As for the confidence thing - at least you can feel superior in making use of community resources and Doing The Job Right when compared to that overpaid underperforming wastrel of a consultant :-)