If they become an atheist they don't believe in God, if they convert to Islam they are now Islam....neither compatible to Christian belief and faith
Actually... Islam and Christianity are extremely compatible.
Christians and Muslims worship the same supreme God, tell many of the same stories, revere many of the same people, share similar mythology, they both believe in Heaven and Hell and Celestial beings and Infernal beings, they both believe in a Devil/ Shaaitan, etc.
They can both be used for mass manipulation, indoctrination, exploitation of the masses, social control, etc... and to justify war and violence and murder and torture and conquest... but they can also both be used to embrace a life of peace, love, compassion, respect, and virtue. It's up to the individual do do what you Will, and believe what you Will.
Islam and Christianity are both Abrahamism, and they can be greatly compatible with each other... if it is your Will to open your eyes.