Who should be a testers best friend?

Answer: The developer.

The person a tester should talk with the most on the entire team is their developer. They should be in constant communication. It does not matter if you are in agile or waterfall development.

New hires, that are new to testing, are so surprised when I tell this to them. They are often shy and afraid to talk with the developer (often intimidated). And the developer is so deep in writing their feature, they don't notice the awkwardness :)

I also, firmly believe the tester should sit near the developer. I have worked in places where there is a separate dev/testing hall (hallway of testers and a different hallway of developers) and I worked at a place where I got to sit beside the developer.

From my experience the amount of information I absorbed sitting near the developer was amazing. I could just listen, or provide real time feedback on discussion that took place, provided valuable feedback on bug fixes (verbally told the developer things they should watch out for), helped to be a sounding board on design decisions. And I know if i said just a little further, the interaction would have been limited to only when i initiated the conversation or when the dev just checked in, or when i found a bug.

For me, I know once I sit in my seat, i hate getting up... its like a context switch and its hard to reorient yourself when you come back to your desk and have to remember what you were doing... (especially if you are like me and have a thousand windows up) :)