https://www.faa.gov/air_traffic/publ...section_4.html
Specifies 2,000 feet minimum between aircraft dumping fuel and another aircraft below midair.
With respect to ground, distance is greater for complete vaporization.
CAA (UK) states 4,000 feet above ground in summer and 7,000 feet in winter.
https://publicapps.caa.co.uk/docs/33...il%202015).pdf (see page 305 of pdf)
This must have been a pretty extraordinary emergency