Depends on what he finds out. You can call him to the stand if he gets pictures of things and he can state he took the pictures to get them into evidence. If he can find drug use, then you can ask for drug testing. What kind of things do you think might show up? Has the PI ever been in court to testify (will he be a good speaker for your case)? Many times you can use the things you find so you/your lawyer can go to the other side and say..."hey, look what we found out now...lets negotiate or we go to trial".
Not sure what you are paying this guy, but most of what they find you can do yourself (unless you need someone to follow her and find out things...)
Great job getting date pushed off!! Did they put a holiday schedule in the meantime?