In the first seasons she did, then the distance grows, after season 5 she gets more and more self absorbed, in the comics, she is at times very distant towards Buffy, even closer to Dawn than to Buffy, in the beginning of season 9 she acts awfully towards Buffy because of the seed of wonder, even if she says she is not mad. She also regrets to have saved Buffy in season 6, she says she has chosen Buffy over Tara to Kennedy. That is not really like friends should be. But then by some subtext I suspect that she might have more than friendly feelings towards Buffy, would explain why her behaviour colludes with the time after Tara and after she came out as gay.
Nevertheless she is not what a friend should be like in later seasons, regardless of her motives.
I agree that both aren't as close as they used to be which is mostly because they grew up and apart imo (Well, Willow grew up. As far as S10 went, Buffy has to make up immense leeway). For me, their friendship isn't crucial for the fandom anymore, hence my wish of a separate Willow comic since her development is also grossy neglected in S10.
I think you see the whole matter a bit one-sided. Friendship is a two-way street and not only how Willow should behave. There are plenty of occasions where Buffy was a bad friend to Willow.
S04 - Something Blue comes to mind. Willow was wallowing in self-pity over Oz' departure and Buffy was mostly irritated/occupied. In the same episode Buffy pines for Riley and whines over Angel (again) while Willow patiently and compassionately listens. When the spell is lifted, Buffy immediately goes back to pining for Riley again instead of setting Willow straight about her screw-up (friendship is also about challenging, not only agreeing and condoning).
Not telling the Scoobies about Angel's return was on Buffy and showed no respect towards her friends, which Giles rightfully pointed out.
Leaving Xander and Willow in the hospital after Xander lost his eye is also not very
friendly.
Willow isn't really fair to Buffy at the beginning in S09, I agree, but Buffy is quite indifferent about the whole matter which is the real (and rightful) reason that Willow's pissed imo. And surprise, it turned out Willow was right. I'd call Buffy a bad friend when she makes Willow feel guilty because the latter had left to bring magic back. And what does Willow do? She caves in and apologies.
I don't see her realization of choosing Buffy over Tara as a proof of being a bad friend. She regretted that Buffy had to suffer but never that she did it in the first place. You can read that in A&F S09 when she says that to Angel.
It looks like that I think Buffy is overall the bad friend here which I don't (expect in S07 & S10 maybe, but that's due to bad and ooc writing imo). I just wanted to even the scores a bit.
And about Willow being self-absorbed? Some people seem to confuse selfhood with independence and just prefer a Willow who is Buffy's unconditional pupp- *ahem* supporter in everything without an own life or purpose. She was self-absorbed in S06, yes, but even there were episodes where she acted selfless and compassionate (between Wrecked and Villains). In all other seasons she is not more self-centered than Buffy or Xander but S06 seems to stick on her.