GFCI with a light switch off the load trips when switch is off
When in doubt, don't use LOAD

Generally when people fit GFCI+receptacle combo devices, what they look for and expect is for this receptacle to have GFCI protection.  They don't have any expectations or plans to protect other stuff also.  

So leave LOAD alone.   LOAD comes with tape across it, and the tape roughly says "Do Not Use.  Wizards Only."   Leave the tape on.   If you need to land 2+ wires, put them all on LINE -- either by pigtailing or using the "screw to clamp" feature on some GFCIs.  

What the wizards know is that LOAD has a very specific purpose: to extend GFCI protection beyond this receptacle to other devices. That may seem like a no-brainer, "everything's safer with GFCI", "what could possibly go wrong" but it means any of those devices can also trip the GFCI and you are stuck with that, even if it sucks.   So wizards do not extend protection lightly.  They do it precisely where they mean to! 

Bathrooms and LOAD

Bathrooms are the worst for this because a) wiring often has a switch and receptacle adjacent, and that invites a neutral imbalance error;  b) you have lots of old, damp machines in bathrooms that are grounded and aren't dangerous, or that are old fluorescents; and c) there's a safety issue of failure of a dangerous tool tripping the light also, and plunging the tool user into the dark, whether it's a circular saw or curler, it's equally bad.  So there's an arguable case in bathrooms for not protecting more than the receptacle.  

Often the use of LOAD terminals boils down to "I still have 2 extra wires, and I see those 2 extra screws to attach them to".  That's using LOAD lightly, just as the wizards warn about.  

So I would absolutely not bust up their drywall to mess with cable runs.  Just remove everything off LOAD on that GFCI and put it on LINE.  At that point nothing can trip the GFCI except things plugged into the sockets.