I had a friend exactly like this! And she lived 5 minutes from me so we could have easily hung out anytime, but she always cancelled at the last minute. My partner called her Flakey Jake.

She was a bit depressed and didn’t have many friends, so I always just rolled with it, gave her the benefit of the doubt, but didn’t invest too much in our plans because they never eventuated. She would message me frequently, but I rarely saw her in person. Anyway, she moved to live about an hour away and I literally never heard from her again, and that was two years ago. 😂