I had an SB in Philly who I loved dearly, but she would constantly do this regarding a second date during the week.  Our scheduled dates were fine, but then she would say well I might be able to come by the day after tomorrow.  I'll let you know. The day of it was always, uhm let me see how the day goes but I definitely want to see you.  Then as it got close to the evening it was I am going to be a little late if that is ok with you.  Finally, it was I couldn't get away like I thought I was going to be able to, sorry babe. lol

After about 7 or 8 times, I finally called her out on it.  I said don't promise things you know you aren't going to do.  She was upset when I pointed out that she was 0 for whatever when she said she could come by a second time during the week.  She made a conscious effort after that to prove me wrong. lol.

I told a habitually late SB that if she arrived +/- within 20 minutes of agreed time she could have an extra $20 ! She obviously didn't need $20 but she admitted she loved the "challenge".   It worked every time. Once she was late by 25 mins and i refused the $20. She didn't argue.

I can't believe you have a SB who really cares about 20 bucks. LOL