Answer: They could be. This is a question that has caused some confusion for a lot of people. Because the Muslims worship one God who created all, it seems that they must at least be thinking about the true God to some degree. Of course, because the true God has also revealed himself as a trinity, to a large degree they are missing out on aspects of who He is. If a person who professes to be a Christian can be helped to understand that we believe Jesus is God, as is the Father and Holy Spirit, and that Islam’s God, Allah, denies this, it might help them realize that to a degree we are not equally informed about who God is, and to that degree the Muslims do not worship God as truly as we can.
I think, however, that it does not help us to reach Muslims if we make it our task to show them that they do not worship the same God. They do, they just do so inaccurately. We and they are better served by helping them to see how our lives have been changed by the grace of God and by challenging them to look at what their way of salvation leaves them with. They cannot have great confidence that they will be accepted by Allah if their salvation is dependent on good works and obedience. Loving them and showing them the genuineness of our lives in Christ is the first step to getting them to reconsider their theology.