When I play Spotify through my phone and my daughter does "Spotify Connect" for Amazon Alex, my phone will sometimes play her song and if I restart the song on my phone, Alexa will play my song. Also, when I'm driving, Android Auto shows what she is playing but there is no sound. If I press pause within Android Auto, the song will stop playing on Alexa. My account is setup on Alexa but my duaghter says "Spotify connect" on Alexa and connects using her phone.
The account being used, is the account you have setup in the settings of the Alexa app. If this is your account, it's possible that you can't listen to music when the Alexa speaker is in use. Your daughter will have to switch to her account via the Alexa app. If you have a Premium for Family subscription, you could try creating an extra account just for Alexa.
Hope that'll work 🙂 Let us know if you have further questions.