One, I would point out verses such as:
Jesus said to them, "Truly, truly, I say to you, before Abraham was, I am." So they picked up stones to throw at him, but Jesus hid himself and went out of the temple. (John 8:58-59)
"I am," or
ego eimi, is the proper name of God, the Greek translation of the Tetragrammaton YHWH (or, for your JW friend, "Jehovah"). Why would they pick up stones to stone him? For blasphemy. Contrast what the New World Translation says: "Jesus said to them: 'Most truly I say to you, before Abraham came into existence, I have been.'" Why would the Jews wish to stone him, then? Claming to have pre-existed Abraham might be very odd, but it wasn't a crime worthy of death. On the other hand, claiming divinity for oneself
was worthy of a stoning, as we see in this passage:
[Jesus said:] "I and the Father are one."
The Jews picked up stones again to stone him. Jesus answered them, "I have shown you many good works from the Father; for which of them are you going to stone me?" The Jews answered him, "It is not for a good work that we are going to stone you but for blasphemy, because you, being a man, make yourself God." (John 10:30-33)
The JW explanation for "I and the Father are one" is that Jesus and the Father are at one in purpose, in desiring to bring people to everlasting life. This of course contradicts verse 33--not only in an orthodox translation, but also the NWT: "you, although being a man, make yourself a god." How is being at one in purpose with God a claim to divinity? Is it blasphemous to merely desire the same things God desires? Obviously the Jews believed Jesus was guilty of something far more serious.
Note also that Jesus doesn't correct them when they accuse him of making himself God. Instead, he doubles down with an
a fortiori argument.
Two, "Jesus never claimed to be God" is a red herring, anyway. Even if he didn't, Christians and JWs both believe the Scriptures are infallible and God-breathed. There are a number of verses speaking of Jehovah in the Old Testament that are alluded to in the New, only they speak of Jesus. For example:
Thus says the LORD, the King of Israel and his Redeemer, the LORD of hosts: “I am the first and I am the last; besides me there is no god." (Isaiah 44:6)
"And to the angel of the church in Smyrna write: 'The words of the first and the last, who died and came to life.'" (Revelation 2:8)
Who is it that died and came to life? Did Jehovah the Father die? No, that is Christ, declaring
himself to be "the first and the last," a title of divinity claimed exclusively by Jehovah.
Jesus is God in the flesh. That is inescapable. Even with the JWs' cack-handed attempt to write his divinity out of the Scriptures, you could use their own Bible against their doctrine.