If your daughter’s confessor told her, “Nobody can tell [you] to go to confession,” then that is unfortunate. Bishop Fulton Sheen once told the story of how he literally
shoved someone into a confessional who needed to confess (
source). I don’t recommend doing what he did, but the point is that it is inaccurate to say that “nobody” (which would include those with spiritual authority like a priest or a parent) can “tell” a person to go to confession.
What the priest should have said is that “Nobody can
force you to go to confession.” (Even Bishop Sheen likely would have agreed with that, and certainly would not have prevented the person he shoved from leaving the confessional if she had continued to refuse to confess.) While those with the authority to do so can encourage and even exhort someone to go to confession, each person must freely
choose whether or not he will confess.
I should caution you though to be careful about deciding that you know your daughter has committed a mortal sin. Mortal sin requires grave matter, adequate knowledge, and deliberate consent. If one of those elements is missing, there is not a mortal sin. You can know that something is grave matter, but only your daughter, perhaps with the assistance of her confessor or spiritual director, can determine whether or not what she did met all of the conditions for mortal sin.
