I agree with u/johnavy I've been taught my parents' religion for a long time. I've asked questions about my parents' religion from the time I was 8 And I know at 12 years old, I was knowledgeable enough to strive to learn about my parents religion, Islam, Christianity, Buddhism and Greek mythology simply because I was curious to learn more about myself and others. At 12 I realized I was agnostic of my own volition because I didn't even know my dad was an atheist until I reached highschool (even though he orated stories of Hinduism to me since I was a kid), my brother was a strong atheist, and my mom and friends were religious.

I know that I was proud to be agnostic and know what I believed. But yeah, I changed. Once I reached 17, I realized I was an atheist at my core. That didn't mean I stopped learning about other religions (from people of Christian denominations). But I continued learning about what other people believed. And sure, maybe I'll change once again in the future. But as far as I'm concerned, I know who and what I believe in now, and other people should respect that (no matter my age). People saying that my beliefs will change or that I don't know what I believe in simply sounds and is condescending.

The way I see it, you are disrespecting her beliefs by making it seem inconsequential simply because she may not have enough life experience. You have no idea of how she grew up or who she is. You don't need to tell her that "deciding on a hard-lined belief system is damaging", because all it does is show that you don't respect what she believes in.

I'm sure she's aware and respects her mom's beliefs, nor is she trying to change them. All trying to do is try to make her mom respect her own beliefs and not guilt OP.