One of our legacy jobs has IR sensors indoor and out. When it gets bright and sunny outdoors, the sensors indoors stop working properly.
Can I put diodes inline with the signal to prevent interaction between different sensors?

Diodes just prevent voltages from traveling in one direction. This will do nothing for your set-up.
The problem is the sun and the outside receiver being flooded with IR, causing the signal from the indoor receiver to get lost. They make special receivers to deal with sunlight (expensive) but if your receiver outside is in direct sunlight, this may not even help. If it's in direct sunlight, you need to move it. If it's not, you may need to try a sunlight proof receiver (example: Xantech 291-80)

I would hate to do this, but would consider it since I see no other solution: provide a switch that the customer can reach that disconnects the signal from the outdoor sensors from the entire IR system. That switch can be left off most of the time, and only turned on when they want to control from outdoors (and the sun is in the right position).

Well, there are a couple of other solutions: replace the IR sensors with keypads or convert to RF (probably too iffy).