Here you go from the Muslim person I know:
Yeah. Slacks are fine. The idea is modesty. If she will be attending the services at the mosque before heading to the burial site, she should wear a shawl around her hair, loosely over the hair is fine.
Just a quick rundown of what is going to take place at the mosque. It is very simple and quick. They will bring the casket in and everyone will line up in rows, similar to how you’ve seen Muslims pray on TV before, if you have. The leader of the prayer will say ‘Allahu Akbar’ 4 times (God is Great). In between each Allahu Akbar he and all attending will silently say the prayers that are required during this time. Thats it for the services at the mosque.
At the burial site it is simple as well. The casket is usually lowered while everyone is there. A member of the family will be the first to toss dirt on the casket. A quick prayer will occur here as well.
Thats pretty much it. Muslim funerals are rather uniform but sometimes families toss cultural practices in there as well, not often but sometimes. Depends on how religious the family is.