Is this the wrong approach?

A girl I was friend with had recently completely lost the plot, everything she’s been doing and saying has been absolutely totally out of proportion.

I had plans to go away at the end of the year with her and a bunch of mutual friends. I left these plans after she lost it at me for no reason, she was extremely illogical.

But I’ve invested $200+ into this trip already (I only get paid $150 a week so it’s a lot).

I know she won’t listen to me if I try talking to her. Would it be wrong of me to speak to her mother? (My friend is 17 and lives with her mum). All I want to say is that for the sake of our mutual friends, I want to know if my friend is willing to be civil with me for our trip. That I’ve invested money and to me it is a significant amount. I’ve had messages from the people going saying that they really wanted me to be there and it’s made me rethink my approach. I get along very well with her mother. As I’m a bit older than her, her mother trusts me to supervise her at parties and she generally won’t let her go places (far from home) without me there. I know if I speak to her mum, the message won’t be misunderstood or relayed incorrectly or biased to my friend.