Can't send picture messages to single iphone user friend

[AC Question]

I have one friend using an iPhone that I can't seem to send pictures to from my android phone. I've tried sending them from a nexus 6 on T-Mobile and a nexus 6p on project fi. I am able to receive pictures from her. This isn't just recent either. I'm out of ideas on how to fix this. Has anyone else had a similar issue? She also said that she's able to receive pictures from other friends with android phones.