Is a 18 inch penis too big for a 16 year old your girl friend says it is?
No male has an 18-inch penis no matter how the male measures it! The average size is 5 and 1/2 inches. No female would be able to accommodate a penis much longer than 6 to 6.5 inches.
I highly doubt you have a 18 inch penis and yes, it would be to long for any female's vagina.
If she tells her friend does she like you but then you send your friend to ask her if she likes me and she said no does she like you?
If a friend says to the girl you like that you like her and she says nothing to my friend and says to him something like I know Im irresistible what does it mean does she likes me back?
First of all she is with your friend so she is off the menu. You shouldn't do that to your friends because you will lose them and when this girl is gone you will be alone. If you value this friendship you will tell your friend what happened so he can get rid of this girl and be prepared to have a falling out with him.