I have a friend who is increasingly annoying me, not because she can't join in on plans or do things with me... just that she can't say no to my face. Other mutual friends have noticed this about her too.
She can never say no to plans. She'll agree to your face and then quietly back out at a later point.
She will go along with joint plans right up until the very last minute, and then think of an excuse. We spent 2 weeks as a group discussing plans, found the perfect deal that required X amount of people. Now she has said she can't come because her sister is her appendix out... a whole 2 months before we were to go. Meaning that our plans are now in tatters as for politics A now want come because B is not etc.