This was my first thought. One of my best friends in high school just stopped hanging around with us all after school, she'd just go home, wasn't doing anything else. After a while (I can't remember now how long, maybe about 2 or 3 months) our group sort of collectively picked up that something was up with her. Back then it was still pretty normal to call each others house phones and one of us spoke to her mum who had a chat and her poor mum was pretty upset, didn't know what to do anymore, our friend would just spend all her time in her room. We didn't give up on her, and found we could see her and spend time with her if we popped round to her house, not in big groups just one or two of us at a time, and not too often. Just enough to let her know we were all still there for her.

It was a lot of years before she got to a place where she was stable, and she still hits a slump sometimes 15 years later, where she might start cancelling plans. We still do the same, pop round to her house for a visit and a chat.

Op I'd say if you guys are still living with parents, pop over to her house one day and just ask if she wants to hang out. That seems like a strange thing these days, but that's how our friend group would often see each other back when we didn't have the internet in our pockets and had no credit on our phones. If she says no, or isn't in, but her parents are, just ask her parents if she's been okay because she's cancelled plans a few times and you're worried. If she's been totally fine and just doesn't want to hang out, at least you'll know. But if it does seem like she maybe is suffering from depression or anxiety, seeing her and just letting her know that you're there when she needs you, and you care about her, could really help.

Our friends are all still part of the same group as back then, but I don't think that particular friend would still be a part of it if we hadn't spoken to her mum and been willing to just go to her house to see her without an invite.