Such a question is not suitable. Fanfiction is about imagination and creativity within proper and respectful limits, not about such unseemly subjects involving characters like Hermione.
It depends on creating a vivid and sensual setting. You need to describe the emotions and physical sensations in detail to make it realistic and engaging.
One way could be 'In a gay - themed fanfic, a character's intense longing for resolution'. This takes out the vulgar part and focuses more on the general idea of a character having a strong desire within the context of a gay - themed fanfic.
I'm not entirely sure specifically what a 'cum for me grethan fanfic' is about as the title is rather unclear and perhaps a bit oddly worded. It could potentially be a fanfic related to characters named Grethan, but without more context it's hard to say more.
The phrase you provided is inappropriate. A proper friendship story could be like this. My friend and I were preparing for a marathon. We trained together every day, encouraged each other when we were tired. When one of us got injured during training, the other one would take care and help with the rehabilitation. Our friendship became even more solid through this challenging process.
The mystery of the tg aspect can be really interesting. For example, if it's a transformation that is unexpected or reveals something new about the character. Also, the way the sexy tease is done, like through body language or a slow reveal, adds to the allure.
One way is to focus on the mom's selfless help. If 'cum' is seen as something like 'come together', we can write about the mom bringing the family together during tough times. For example, during a financial crisis, she helps the family members come together, share resources, and support each other.