Husband nose popped when flying today, immediatrly followed by a yellow water like liquid drain for about a minute? seems fine now but strange

CSF leak: This could represent a leakage of cerebro spinal fluid, which can sometimes occur spontaneously.

Simple physics is: ...at work here. Even tho the airplane cabin was "pressurized", the air pressure is generally lower at maximal altitude than it is where you live. Low atmospheric pressure causes the air in the nose or sinuses to expand. That, in turn, can force any liquid that was also in there to drain out. Unless he gets thick, nasty drainage, headache or other symptoms, no worries!