I think its great you are doing Relay for Life! It will certainly help her out and encourage her when she sees all the survivors when they do their first lap aroudn the track together.
My mom always participates with our church, we have a HUGE group that does it.
Are you going to try to stay the entire night? If so you may want to bring a jacket, it always seems to get chilly about 2am
Also either bring some snacks or a bit of cash. At ours several groups will usually have grills going and sell food to others as part of their fundraising.
Take a couple chairs so you can rest when you get tired
If I think of anything else I'll let you know. I never have the time to participate, but I always go out for at least a bit to support mom.