if its currently working 'as is', then its probably best to leave well alone. however by not defining relationship (after selecting the enforce relational integrity option) then you don't know what state the data is in.
there is nothing magical about RI, it can be done in software rather than the DB. it is however far easier to enforce RI through proper use of relationships within the DB that it is through code.
the original author and users of this system, for wahtever reason, don't give a monkeys about the qulaity of the data inside the DB.
attempting to enforce RI relationships after the event is begging for trouble as before you can apply a relationship the data must be in a form that can accept the new constraints. so my advice would be leave it well alojne. if you have the option void this db like the plague and any other steaming piles of ... that author has created
I'd rather be riding on the Tiger 800 or the Norton