Thanks for the question.
- In some sense her culpability is mitigated for the missed Masses, but has she made the effort to go church?
- Does her mom prevent her?
If the answers are yes to these questions, then it is probably OK for her to receive Communion.
If she isn't making any effort, however, then she bears some responsibility (though standing up to her mom may again mitigate her culpability) so the answer is a little subjective.
It is likely a venial sin and not a mortal sin because of the circumstances but, in the long run, God is calling her to be an independent young woman capable of her own decisions and responsible for her faith life. It would be worth having that conversation with her.
Confession is a good idea anyway, since it is a good habit to go on a regular basis (at least monthly).