This is a way to express the acceptance of smoking pot or accepting somone who does so, without overtly mentioning pot or marijuana.
You may find the phrase 420-friendly often in personal ads, especially on Craig's List.
It refers to the term 420 which has become lingo smoking pot.
See the definition of 420
"420 originated at San Rafael High School, in 1971, among a group of about a dozen pot-smoking wiseacres who called themselves the Waldos"
"Single white male, 420-friendly..."
"Hey, is this party 420 friendly?"
One who specifies and arranges a date with someone who is cool with pot smoking to the point of excluding those who do not condone the practice.
Seeking woman, tall and smart. Must be well read and 420 friendly.
You better be careful, those highway police are definitely not 420 friendly. They put their nose right up to you.