It can be hard for cis people to understand the difference in what gender truely means. To many, gender and sexual attraction are fused. They don't get gay people so trans people are more outside their realm of experience.
For some like your friends mom, she thinks that helping her daughter transition was so she could live with a man and not be seen as gay. Her mom does not understand that quite simply our minds are wired with certain thinking patterns and instincts that make us female or male and that is completely seperate from our sexual attraction wiring.
Perhaps if she can show her mom an explaination she could come to understand it. I have a ton of really scientific information but it is beyond a lot of people's understanding as well.
Maybe her mom will just think about it if she explains it like this:
Our mind's gender is formed in the second trimester of life. Our brains form in female or male patterns by then. The way we think, the way we feel emotion are all "hard wired" in us by then. There are deep instincts that cannot be undone. Desires of socialization with others that are very different between genders. This is seperate from sexual attraction.
Sexual attraction is set toward the end of the third trimester. This is what makes us find masculine or feminine body forms attractive to us.
From some one who grew up when people left me no choice but to conform, I can say that trying to fit in with boys and men never felt right. The way they think and feel doesn't make sense. Have your friend give her mom this info if she wants it. Ask her mom to consider the way it feels when she is the only woman standing around with a bunch of male coworkers, except imagine not only the feeling of exclusion from the conversation but also what guys say when there are no limits. When they see no women around for them to be concerned about insulting.
It does does not feel very good most of the time. Then often women exclude you from their conversations because they see a man not another woman. This is even more painful because what limited female conversation you do get included in leaves you feeling good inside. So it is a life of feeling excluded and lonely. Simple things like friendship are hard to come by. Then there is that constant nagging feeling that you are supposed to be different. I fought this for so many years. The deep background anxiety. The wrong hormone for your brain's wiring creates a sensation that is impossible to describe to those who have not suffered it. We use the word "dysphoria" because it means the opposite of content or happiness. Tell her mom to imagine this for a lifetime, this never ending mental anguish.
I liken it to my conciseness having to operate in way that my brain's operating system was not designed for. Every day is another day of faking it. Doing it because of fear of being real, a fear of others discovering the real thoughts and feelings in my mind. Always having to hide my emotions because guys don't feel the same ways and guys don't ever cry. It is a life of internal torment. This is what awaits the life of a trans person who has to hide who they are. I did this for 40 years myself.
Please copy this and give it to your friend. Tell her she is free to share this with her mom. Maybe, just maybe, this will make her mom understand. I just can't stand by while a young transgender person on is forced into this same suffering my life was. Share this freely with anyone you meet whose family is on the fence about supporting their person in transition. If the person knows they need it, then believe them. It is real.
Josie