Can atheist and believers be best friends?
Sure. My late 2nd wife was a rural Methodist and I deconverted on her watch. It was of no big concern to her, and no threat. That's unusual, but then, she WAS special.
I think any relationship that's based on things besides church activities, theology, etc., can thrive independent of differences of opinion about religion. The world is MUCH bigger and more diverse than that. My late wife and I were both software developers (though she was no longer active due to disability) and had many interests in common, like hiking, travel, science ... I think the key is to find someone who, whatever existential beliefs they hold, do not confine themselves to being tribal around them.
OF course they can, I am an anti-theist my wife is a druid, I think she is stupid in that respect she thinks I am ignorant, but other than that we have a 27 year marriage, two kids (one is an atheist one is a born again) four grandkids and a plethora of mutual interests.
The problems only come when you make your religious/spiritual/ideological beliefs the centre and the whole of your life, which frankly is a waste of 99.9% of your life.