Muslims should be kind and gentle when dealing with all people, including non-Muslims, so that they will have the chance to see the true Islam through their practical good behavior. Therefore, if a non-Muslim, out of respect for Muslims or in order to learn Islamic prayer, joins them in salah, he/she is allowed to do so.
Dr. Muzammil H. Siddiqi, former President of the Islamic Society of North America, states: Muslims must pray with wudu’, but if a non-Muslim, out of respect for Muslims or in order to learn Islamic prayer, joins them in salah, he/she is allowed to do so. This may be a way for this person to come closer to Islam, and Allah may lead him/her to the right path. Also, as a rule we should know that if a Muslim prays without wudu’, his/her prayer is not valid, but it does not invalidate the prayers of those who are standing in the same line with him/her.