My Female friend proposed me?

Am 17(M). I know this girl since class 9th. She used to be my crush and I had proposed her but she rejected me saying she just wants to be friends. Last year after my school farewell we talked once in a blue to wish birthdays and all. Some months ago I got to know that she rejected me cuz she was and is already in a relationship.

Today out of no where she messaged me asking if we could be together. I knew something was wrong and called her. She was crying. I got to know she again got into a argument with his boyfriend's sister. And when she complained about this to the boy he didnt said anything.

I consoled her and just said he may doesn't want talk about that topic. You just talk about something else or wait for him to reply as he might be busy. (she has a medical condition due to which whenever she cries her chest starts to pain). So as a friend I just helped her lighten her mood. She said thanks that's the reason I started liking you. You care for me more than him. I didn't replied to her and gave a excuse to hang up.

I don't know what to do...

Edit:- Considering your advices i have decided i wont be a rebound.