There is not much room for that in Jehovah's Witness teaching. With them you have to earn God's love by your behavior, evangelism, etc. So begin with her own love for her family, and your own commitment to loving others. You can then suggest that God must be more loving than we are.
If she really wants to argue about the Trinity, you can point out that
you cannot love alone. . We believe that the three Persons of the Trinity
were united by love (e.g. electrons, positrons, and neutrons are a oneness
united by atomic force) long before the world was made. A solitary Unitarian
God can never be a loving God. You will find this explained more fully
in an article about the Trinity
on this web site.