Cassandra respects Ellana's own abilities as a mage, but she'll still be pretty blunt in asking after her qualifications as a teacher, Sina's own level of control over her magic, and whether Sina was acting under her orders/with her knowledge. If she wants to step down from training Sina, Cassandra won't argue, but she'll insist that someone be responsible for supervising her, and probably insist that her training stop until there's some guarantee that this won't happen again. She would argue for Templar supervision, but if she can be convinced that a) this was an accident and b) Sina absolutely will not try anything like what she was doing again, she'll begrudgingly back down.

If/when Sina's training does start again, Cassandra will vote for her teacher to be a Circle-trained mage - she's thinking someone like Vivienne, if she'll agree to it. If Ellana has the names of mages who have been studying the shards in detail, Cassandra wants to hear them.