scairlilgirl, Your best friend is taking a HUGE chance an will become pregnant if she does not stop this.
In a man, cuming is the slang term for ejaculating. Ejaculation is when his semen that contains the sperm comes out of him. This happens when he gets his orgasm.
She is trusting him to have sex with her but pull out before he gets his orgasm. This is dangerous as he can not always control his orgasm and it is unnatural for him to pull out. What he wants to do is just orgasm while he is inside her. One of these days he is going to misjudge it and ejaculate/cum inside her vagina.
When a boy gets sexually aroused (he gets an erection), he has a gland that secretes a fluid called pre-ejaculate or pre-cum. This fluid prepares his penis for the semen and sperm that will follow, to safely pass through. It cleans out any impurities, old sperm and urine left over inside his penis. This fluid also lubricates his penis.
While he is aroused, he cannot pee as there is a valve that close his bladder off until after his erection goes down. His semen with the sperm in will also not come out until his orgasmic contractions will make it come out. So this pre-cum is responsible for his wetness.
So she is taking two chances - She will get pregnant because he will make a mistake, and she is endangering her own health as she can get infections and STDs.
PLEASE ask her to act like an adult if she wants to have sex like adults. She should protect herself before it is too late. She has a lot to loose when she gets pregnant, he not so much.
If a woman says "I am cuming", it means that she is having and orgasm and it is normally done to tell her partner to continue what he is doing and not stop. No ejaculation, leaking, gushing, peeing or squirting is implied in this term for a woman.
Take care!