Islam orders that parents be treated kindly and graciously even if they are non-Muslims. Allah says: "Your Lord has decreed that you worship Him and that you be kind to parents" (17:23). Islam also exhorts people to observe and maintain good relationship with kith and kin.
The obligation of kindness and good relationship is emphasized on the occasions of joy and merriment as well as on the occasions of difficulties and afflictions, the greatest of which is death that brings relatives together when they are bereaved of one of them. Man intrinsically tends to express his feelings towards the deceased, whether a relative or a close acquaintance. Islam calls for respecting man, whether a believer or a disbeliever, in his lifetime and posthumously.
Therefore a Muslim may attend the funeral of his non-Muslim parents or one of his non-Muslim relatives. He may attend the religious ceremonies held for the deceased in churches and synagogues, provided that he should not participate in the prayers, rites and other religious activities. He may also attend the burial process. In all that his intention should be to do the duty of kindness (to parents) and good relationship with kith and kin, and sharing the misfortune with the family and strengthening the relationship with relatives and avoiding what may lead to estrangement if he fails to attend such occasions.