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

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.