I'm pretty sure the (UK) rules specify that no disabled person or person of reduced mobility is allowed to occupy a seat where they are impeding the exit of another pax - i.e the disabled or obese pax would need to occupy the window seat.

I have, on one occasion, quietly spoken with the cabin crew and advised them that i was not comfortable sitting on the window side of a hugely obese pax (occupying both the seats between me and the aisle) as it would restrict my exit. They understood and under a pretext to prevent embarrassment to either myself or the other guy, moved me to another seat.