It is definitely possible.

What is good friend? Good friend (Best friend) means a very very close friendship. So, if this happen between a guy and a lady, they are in friendship. They are not fall in love. They won't be jealous with each other in each party love relationship, and they will encourage each other to live happily with their partner they will wishing each other all the best.

I don't think they will have sex relationship also......because most of them not Dr. Chua and his "best friend".

This is luck. Whether you meet a very very close friendship in opposite sex. I don't know any special words for this kind of relationship in English however in Chinese could say is 红颜知己。

I believe with that. Because it happen to me.....I think I'm lucky.