If you are talking with a Protestant, try the following approach.
Refer to the Parable of the Talents, Matthew 25: 14-30, especially verses 21 and 23.
The Lord promises us that in heaven, His good and faithful servants will receive rewards, specifically, mansions and crowns.
Everyone will receive a different reward. We will not be jealous over or covet someone else’s crown. The exact nature of our reward will be up to Christ.
Many of us speak of our loved ones going to heaven and walking on golden streets, or living in a mansion, or being free of disease. We really don’t know any of this for sure, do we? We are making statements about our deceased loved one based on what we infer from the Bible. We infer that the Lord will grant our loved ones who were His disciples a reward, a crown, and a mansion. We can’t point to a specific place in the Bible where it says, “Grandma will receive a crown in heaven,” but we know Scriptures that imply that Grandma will receive a crown.
There are passages of Scripture hinting that Mary, the Mother of Jesus, received a “crown” in heaven. (Revelation 12 is especially graphic.) There are quite a few Old Testament passages implying that she will be/is the Queen Mother.
Whether you accept this “crowning of Mary” or not, it is fair to say that Mary did receive a reward from Christ when she arrived in heaven, just as we will all receive rewards.
Who are WE to question what the Lord Jesus Christ decides to give to His mother?
Who are we to question what the Lord Jesus Christ gives to any believer?
Does it matter to our salvation? No. His death on the cross still avails, His resurrection is still the promise of our new life, His ascension still gives us hope that He will come again. He is the KING of glory.
So Mary’s crowning is not specifically spelled out in the Bible. Neither are your sentimental wishes about Grandma and her reward in heaven, yet you KNOW that Grandma went to her reward.
In the same way, the Church infers from the Bible that Mary was crowned Queen of heaven and earth.
This teaching does not affect the Christian Gospel.
“Believe on the Lord Jesus Christ and you will be saved.”
There is much that we believe that isn’t specifically spelled out in the Bible, but was developed over the first 400 years after the ascension of Jesus. (For example, the doctrine of the Trinity.)
It is possible to infer from the Bible that the twelve patriarchs and the twelve apostles sit as kings in heaven. Does this make Jesus any less the King? Of course not.
Christ will give crowns to many of His servants in heaven: the soul winner’s crown, the martyr’s crown, etc. Will these crowns make Christ decrease? Of course not.
Christ’s majesty is not diminished when He gives crowns to His good and faithful servants, including a crown to His mother.
Keep in mind, too, that the Bible teaches that all of us will cast our crowns at the Feet of Jesus. The crowned saints in heaven, including Mary, never seek royal glory for themselves. They always direct us to the King of Kings and Lord of Lords.
The teaching of the crowning of Mary does not diminish the Majesty of Jesus, it only increases the depth of His love for all of us. Not only does He give us eternal life, but He gives us mansions and crowns.