I dont feel its fair to drop someone if she is not going to do x.
You can suggest what would be helpful for her, but withdraw a friendship if she doesnt accept your advice?
I always was avialble to support classmates who were not as strong socially, academically etc. and I dont regret it. Chessed is what makes us better. Thats what Hashem wants from us.
Now, if you feel its taking too much of you, this is where you can set the limits. How often would you want to spend time with her? you do need to balance it out. you have other friends/family/school needs (lets say twice a week, reasonable? once?) or what topics you are willing to disucss.
You can try to share your mitzva with other friends, make a Kvius with one more nice friend of yours :”lets make x happy and spend one afternoon with her every week, or a Pizza every other Sunday?” etc. its a hugh Mitzva, and the impact on her life will never be forgotton. You are the light she has for now.
Set up your limits, and when you spend time with her, listening to her, giving her your friendship, remember you are emulating God and connecting with him. ‘Af Atta Rachum”
Hatzlacha!