A Muslim funeral is similar to others around the world. It is a time of mourning, it is generally serious and a person's clothes should reflect that mood. There is no set dress code for Muslim funerals, but common sense and an understanding of Muslim sensibilities toward clothing will allow a non-Muslim to blend in and show proper respect.
Dress Modestly
Modesty, modesty, modesty. This is the key concept to keep in mind when planning funeral attire. According to the Sahih Al-Bukhari collection of Muhammad's sayings, book 72 verse 683, Muhammad once said: "Whoever drags his clothes (on the ground) out of pride and arrogance, Allah will not look at him on the Day of Resurrection." (See References 1) Dragging clothes on the ground, in seventh-century Arabia, was a way to flaunt wealth and project status. When choosing your clothes, make sure you are not trying to draw attention to yourself.
Men
Proper attire is easier for the male non-Muslim than his female counterpart. Wear pants that don't expose skin. Wear a shirt that covers you up to the neck. Wear your hair in a plain fashion. Don't wear anything tight. Clothing should allow you to blend in rather than stand out – this means no bright colors and no flashy jewelry.
Women
Women need to wear something loose-fitting that covers all but the hands and face. Like the men, women should wear something muted that does not call attention to the individual. Women should not wear jewelry and other flashy accessories. Do not wear lipstick, eye-shadow, mascara or other things designed to increase sexual appeal. Hair should be simple. Bring a headscarf as well in case the majority of the women wear one.
Call ahead
There are more than a billion Muslims on earth, and every one of them is different. If you want to assure that things will go smoothly, call and ask a family member of the deceased. Western Muslims are used to explaining Islam to people, so they won't be offended when you ask. Many mosques in the United States provide headscarves for women as well, but you'll want to call and make sure.
Show Respect, You Will Be Respected
Book 23, verse 398 of the Sahih Bukhari collection of Muhammad's sayings relates a story where a funeral passed by Muhammad and his followers. Muhammad stood up respectfully and continued to do so after his followers noted that it was a Jewish funeral. Muhammad stated plainly that one stands up, respectfully, for a funeral.