You're not overreacting!!!!
If someone sends this kind of images of herself to the other person then she doesn't respect herself.
Why? Maybe she has been sexual abused or if I look to her mothers reaction. She has been taught that it's normal to do these things. Which in my opinion is also a form of abuse or child neglect.
Sending pornographic images of herself to others and maybe more, like having sex with others. Could have some very disastrous consequences.
Someone could post these images online. This is illegal, but it happens. There's a lot of child pornography to be found online.
Sex:
Catching curable/incurable STI's and some of then can even be deadly. Like HIV, hepatitis b, warts that causes cervical cancer and cancers of the vulva, vagina, penis and anus.
This last one is given to the other person by skin to skin contact. Even if someone uses a condom you can still catch this!
There's always the change of an unplanned and unwanted pregnancy.
Changes of meeting the wrong kind of people who'll just use a girl like her for sex. Girls who were being thought that sex is a normal thing to do with others. Have a higher change of becoming a prostitute.
These changes for someone like this girl to end up like that will only be higher if she doesn't finish her school.
This is very serious, but I've no idea on how to deal with it. Clearly her mother is no help at all!!!
Maybe have a chat with the girl. She probably doesn't understand what she's doing.
That it's wrong and illegal to make these pictures of herself. What if they fall in the wrong hands?
That having sex is fun, but also very dangerous. Around 900 women die every year of cervical cancer. 70% of these deaths are being caused by a wart virus.
Don't scare her either.
But let her know that her body is her body and it's not something she should so easily share with everyone.
Just be nice for her. So she feels she could talk to you. She might be a victim of sexual or other kind of child abuse. It might also be that her mother is very poor in her parenting skills.
If you're there for her and she feels safe with you. In case something is seriously wrong with her. She could have a talk with you about it.
It must be a very difficult situation to find yourself in. Good luck with this poor girl and her mother.