To think this is due to ADHD?

But it sounds like what she's doing/you're doing are the same?

You're not offering to get her something, she's asking. I'd assume that when she says she's popping out, she's thinking you'll ask her if you want something, same way she does with you?

Even ND can learn to fit simple routine. She is 100% rude.

Honestly, could be either. Maybe a bit of both. She may genuinely not remember you buying last time, or that might be true sometimes.

It doesn't really matter why she does it, though. If yes annoying you, you can a) calmy say that it upsets you that you often buy a snack and she doesn't reciprocate b) every other time she asks for a snack say, "ok can I borrow your card because I got the last lot" c) start saying "no, I think we should stop treating each other its hard to keep track" or d) do nothing and keep seething . All of those are perfectly polite and don't depend on attributing the behaviour to her ADHD.

I wouldn't put it down to cheekiness, you just communicate differently. She asks directly, so expects you to ask directly if you want something.

If she's ND it might explain why she's not picking up on your method of communicating, if there's one.

Try asking instead of harbouring a grudge, it's better all round.

It sounds like their is more about inattentiveness and self-focus rather than deliberate selfishness. ADHD can definitely contribute to this—people with ADHD often struggle with social reciprocity, not because they don’t care, but because they’re hyper-focused on their own immediate needs and impulses. When she asks you to get her something, she’s in that moment thinking about what she wants, but when she goes to get herself a coffee, she’s likely just focused on her craving and not making the mental connection to offer you one.

That said, ADHD isn’t a free pass for everything. If this is happening repeatedly and she’s aware of it but still not making an effort to change, it might be worth addressing directly. Next time she forgets, instead of joking about it after the fact, you could try preempting it: “Oh, if you’re going out for a coffee, I’d love one too!” and see how she responds. If she still doesn’t get the hint, then maybe your OH is right, and she’s just a bit of a CF!

No she just sounds rude. I would stop buying her coffee and a snack if she’s too tight to ever get you one

Occasionally I could forgive, but if its every time? I’m thinking shes a CF.

What do you say when she says she’ll pop back out and get you something? If I were you I’d always pull her up on it and when she says “oops, I’ll go back out” say yes thank you, I’ll have a latte and a muffin.

Don’t let her get away with it and then you’ll see if she’s hoping to get away without reciprocating or she’s just genuinely in her own little world.

Surely ADHD isnt a one size fits all?

Doesn't sound like it to me, sounds like selfishness