I have friends with ADHD, I am married to a "diagnosed in late 40s and now evangelical about it" ADHD man, and my teenager has recently been dx AuDHD. Each are different.
Friends: Individually, I assume nothing. If it's something where I'm dependent on them (eg they've promised to bring the salads for the BBQ) then I check in a week before, a day before, and on the day. But I will always have a backup plan.
Friends: collectively, if we're planning something like a weekend away, or a trip to the cinema, I take responsibility for nothing until I have full commitments and funds in my account. Recently I was booking for the cinema (not a mainstream film, specific day, specific places). There were 5 of us interested, but I booked for 4 who were dead certs. With the one who I expected to be flaky I included her in the group chat, said "I've booked tickets in row x, left a single seat beside us, book that one when you can". She did not book, she did not complain
DH: has always run his life via task lists and calendar items, and always builds in contingency time for things... to a fault. Would rather drive at 6am to a theme park opening at 9am because the drive will take 2 hours than drive at 7am when there's a little more traffic so the drive might take 2.5 hours and the ride queues might be building up inside. This has become more pronounced since diagnosis. It's an ongoing work in progress to maintain my boundaries and be understanding but sometimes I do wish I had a soundproof rage room.
DS (17): basically away with the fairies. Can very easily run his daily life, look after himself. Can plan ahead but doesn't, because it doesn't feel rewarding. This is problematic in terms of studying and homework (I have a whole thread on this). But also in that there are things he wants to do like travel with his mates, but none of them are capable of putting together an achievable itinerary (and consequently, none of us as parents is handing over enough money or god forbid their passports, for them to strike out and get into a pickle.)
In your situation OP, if you think there's less than a full likelihood of her doing what she says she'll do, make sure you've got a fall back plan or that you're doing a thing that other friends are also doing. You get to spend your time well, she either turns up or she misses out.