This is their own website’s guide to prayers:
lds.org/manual/gospel-principles/chapter-8-praying-to-our-heavenly-father
Notice how they refer to “Heavenly Father” even when God, more grammatically correct, is what a Catholic or Protestant would normally say.
Some examples:
“In what ways has
Heavenly Father answered your prayers?”
“As we pray to our
Heavenly Father, we should tell Him what we really feel in our hearts”
“We need to pray to ask our
Heavenly Father for strength to live the gospel.”
"Prayer is a sincere, heartfelt talk with our
Heavenly Father. "
And I am telling you from experience with Mormons, if allowed to lead the prayers, they will ALWAYS begin with “Heavenly Father” and will react to you when you insist on using one of the other traditional opening Prayer statements referring to God. Including the opening line of the Our Father prayer.
The Our Father prayer is interpreted in Mormonism as a “Template prayer”, and is actually rarely spoken word for word in a Mormon church.
Pray with Mormons, by all means. They are nice, quite loving people. But be aware that this is indeed one of their quirks caused by their belief about God’s nature, and that you should lead the prayer if you can(Don’t be rude about it though). Lead them in the Our Father if they let you!