How would you respond to someone who says you should only pray to God and not to Mary and the saints?

In my family, we talk to one another - child to child, child to parent, parent to parent, parent to child, family members to family members.

I will talk to my Heavenly Father, My Heavenly Big Brother, Beloved Holy Spirit, Blessed Mother, my Guardian, and my Heavenly Siblings - those in Heaven and those in Purgatory.  They all Speak to me, too; not in locutions or visits, but in how my life unfolds.  There are some activities/events can can be nothing more than as coming from Them.

It pleases Our Lord in a special way when He Sees that we partake of the Familial Dynamic.  We are part of the Heavenly Family.  We are part of God's Special Love that He has for each of us and for all of us.