I'm going to tell my friend she wasn't sexually assaulted
Why do you think she wasn't sexually assaulted?
She seems very happy now and I just want her to stay that way
Just because she is happy now, does not mean that it did not happen. She shouldn't be expected to be sad about it forever.
I'm going to ask her to stop telling that story and rubbing it in my face.
In this case she's the only one who can really tell what happened to her, it's not your place to decide if it was sexually assault or not. You're her friend so you should trust her. Instead of suppressing it, you need to sit and have a conversation with her.