It's not mandatory to use left foot. You can do it with right too. Her mistake was that the distance between two legs wasn't that much that's why wheels collapse. While doing skating it it important to note you have to maintain the distance between your two legs.
Dada tumi kothay shakow bolo na please ami shikbo karon ki amar kache akta scating shoes acha to ota ami chalate parina to tomar kach theke shiktam please bolo tumi kothay shakow