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"