What shoud i do since she denies it?

i went out with this lad ages ago and i kissed him on the second day, for some reason even tho it was jst a peck on the lips my mate says tht im a slag n she is always goin on about how this person tht she went out wid was a proper relationship thn mine even tho thy neva kissed. the person she went out wid has told me tht he wishes he had neva gone out wid her and tht now he hates her, she keeps sayin it 2 me and i jst want 2 turn around and tell her even tho i know how upset she wud get wht shall i do cos every time i confront her and tell her how upsetin it is she always denies sayin it. i dnt want 2 lose her as a friend but it is gettin 2 much 2 keep it in. what should i do?