A Christian is praying and fasting with the Muslims by way of experiment
Question

                I invited a Christian to Islam, and he started to pray and fast by way of experiment. Is there anything I should do, or should I let him try it out?.
Praise be to Allah.There is nothing wrong 
    with leaving him to try it out, whilst also reminding him of how easy this 
    worship is and the great effect it will have on the soul, if he does it and 
    is sincere towards Allaah, and that it is only prescribed for a great reason 
    and not to cause difficulty for man. Here we see modern science speaking of 
    some of the benefits of prayer.

     He should also be reminded that it is a matter of individual 
    taste and experience, rather it is a serious matter that has to do with 
    attaining Paradise and being saved from Hell. And Allaah is the Source of 
    strength.