Strictly speaking, worship is an act of reverence directed to God or whom or what we suppose is God. The word worship used in other contexts is only a metaphorical usage - like, he's the star of the team - worshipped by fans across the country. Since Muslims say there is only one God and that God is invisible, they would certainly not be worshipping anything that we can see in this world. This being so, if a Muslim comes upon a person worshiping a stone, he would naturally feel that the worship is erroneous because the stone is not God. He would however, have failed to see the following:
1. A person worshiping an idol may have the following reasons:
(a) He understands that the idol, by special processes, has the presence of God in
an intensity he can relate to.
(b) That God is not restricted to the idol but God has given an opportunity for him,
with his limited senses, to grasp His munificence by approaching Him at a focal
point in a temple to pray for blessings.
(c) That an idol is not the be all and end all of his interaction with God - all idols
come with its own traditions and mythologies and incantations and
scriptures and teachings, and temple histories and last but not least, religions.
2. That even though a Muslim is instructed to worship none but Allah, he cannot,
being a normal human being of limited senses, conceive the invisible Allah and
therefore would necessarily have to reduce Allah to the level of his capacity to
conceive the invisible and that can only be in the form of idols, whether by usage
of the word Allah, or Ka'aba as direction marker or prayer at fixed times.
3. That though the form of worship and concept of God may differ between
different religions, all worshippers are human and therefore given the same
circumstances, their experiences are likely to be alike.
In conclusion, worship of God or Gods mean just that - a communion between humans and the ultimate power or powers, which we can never conceive except in human ways. Therefore all worship is necessarily idol worship.