"Be on the watch for the false prophets who come to you in sheep’s covering, but inside they are ravenous wolves. By their fruits you will recognize them. Never do people gather grapes from thorns or figs from thistles, do they? Likewise, every good tree produces fine fruit, but every rotten tree produces worthless fruit. A good tree cannot bear worthless fruit, nor can a rotten tree produce fine fruit. Every tree not producing fine fruit is cut down and thrown into the fire. Really, then, by their fruits you will recognize those men" (Matthew 7:15-20)
That is a quote by Jesus from the Gospel of Matthew in the New World Translation (preferred by Jehovah's Witness members). It is corrupted in many aspects, but this verse is relatively untouched. She may not accept this teaching of Jesus if another translation was used.
Show her this quote and let her see that softly let her understand she is wrong. "By their fruits you will recognize these men"...she is friends with you, and she doesn't think you are bad. So logically, being the fruit (Sikh) of the tree (Guru Nanak and the other gurus), means that Guru Nanak isn't a Satanic false prophet. She doesn't have to accept him as the true guru of Kali Yuga, but she must understand he is not from Satan.
Also she ought to look into the brave actions of Sikhs and the Gurus. For example, tell her of Guru Tegh Bahadur's Martyrdom.
Also please note for yourself that Jehovah's Witness is a false cult. Follow what the post by Nihang96K about Orthorox Christianity. He speaks the truth.
