Muslims don’t believe there IS a divine Son, so we can understand why they think one is not necessary.
We get into a bit of a logical dichotomy when talking about God, when we say God is both merciful and just.
Justice means the punishment is comparable to the offense. “An eye for an eye” is a statement of pure justice.
Mercy means the punishment is less than the offense, or perhaps there is no punishment at all.
So how can God be both merciful and just? The two properties appear to be (and actually are) mutually exclusive.
Are you starting to see where this is going?
If God exacted perfect justice upon humanity, there would be no room for mercy (and we would all be crispy critters). If he exacted pure mercy, there would be no room for justice (and we could get away with murder). Because justice and mercy ARE mutually exclusive. They mean different non-compatible things.
But, if God subjected himself to justice in order to be merciful to us, the logical problem goes away. God the Son showed mercy by taking on the sins of all humanity. He sacrificed himself in atonement for that sin to satisfy justice. Now the finite debt is paid with an infinite payment, and only mercy remains.
Ask your Muslim friend if (unitarian) Allah is both merciful and just (and he will say yes - I guarantee it), and then ask him to explain how this could be. You have the answer - he does not.