YANBU to ask. Well done for wanting to support your friend.
You can try to talk to people if you want to, but no obligation. You might find someone approaches you, often with “So how did you know X?” Just be polite, keep it light.
Don’t take flowers, it’s usually family only.
Strappy sandals will be fine as long as you are comfortable in them. I don’t think anyone will be upset by thin straps in this weather. Personally I think lots of cleavage is out of place at a funeral, so if it’s low cut I would try to find a thin scarf or shawl. Doesn’t have to be black, just avoid very bright colours.
Typically the service would be half an hour or so, unless it’s a full requiem mass in a Catholic Church. Crematorium - 30 minutes max unless they have made special arrangements for a double slot.
Afterwards, the chief mourners leave first, then everyone else files out. The minister/celebrant usually waits by the door, it’s conventional to say thank you, although if there’s a crush, don’t worry if you slip past. Then the family will be there, and you can speak to your friend and (if comfortable to do so) give her a hug.
There may or may not be a “do” afterwards, that may be announced at the service or on the service sheet. If it’s announced, you don’t have to be personally invited, you can just go - if you want to. Likely more time there to speak to your friend.
Hope that helps, try to relax. Funerals are never nice, but the most important thing is showing your support by being there.