I would wear what I have a nice pair of clean jeans ,a nice clean long sleeve shirt and new clean fresh socks(because you will probably have to take your shoes off),my new boots..and remove my hat when entering the mosque or attending the services. If they did not want me to attend this way I would come later and pay my respects. Now if you mean as a traditional dress/garb they would have to provide me a set of clothes they feel are adequate for their loved ones funeral service.from what I have seen they(the men/boys) would wear all white woolen clothes a woven wool shirt and paints with a red sash for a belt and a what I can only describe as a jewish style cap(over their crown or bald spot) for representation of worship respect to their God.so I guess you would have to describe what type of muslims you are talking about.strict religious muslims or today's muslims that are still practicing their religion but not holding to the tradition. The other thing I have learned is they would be separate from the women and girls in most traditional cases and the women would be covered head to toe in what I call a wrap style dress over top of their regular clothes covering their heads.but again this would be determined by how they still follow their traditional religious belief.in other words even if I did not believe as they do I would still respect their ways especially in this case of laying to rest one of their loved ones.
