And, just to clarify, there are no SEATS at the Mural stage (or at many of the stages, actually). It's just folks sitting on the grass. Luckily it shouldn't be soggy that day. If mobility is an issue for your mom and/or sitting down and standing up from the ground is difficult, there are a few benches on the walkway between the stage the Center House food court. There are only about 3 of these, and they don't have a perfect view of the stage (and if folks stand up, she won't be able to see) but she should be able to HEAR just fine. There tends to be a beer garden in this area towards the back...and this might have some limited folding chairs, I'm not sure.
Also, last year, beside the Mural Stage, Starbucks had a special VIP fenced off area, with some chairs and shade. But you had to sign up ahead of time. Here is the link for the free RSVP for this year. http://www.starbucks.com/seattle/events/bumberfan Again, this area is to the side of the stage, so the sound should be great, but the view won't be. But, if seating is important to your mom, this is probably the most comfortable way to have a seat for Russell's show. If she's interested in getting a spot in the center near the stage, arriving towards the end of the act before Russell should be plenty early enough. (And she can use her jacket/hoodie as something to sit upon during the day)
Personally, i think checking out a comedy act and/or a theatre performance is a nice break (also, these venues have actual seating, too). You get a guaranteed ticket to ONE comedy lineup. You have to find the comedy ticket booth (it's generally at the north end of the Center House). When you get to the front of the line, you get to choose which act you'd like (Based on availability. If one particular big name comedian really appeals, she'll need to get there as the gates open to get a ticket before it sells out). Then, just be sure to be in line at the comedy theatre 30 minutes before her assigned show starts. It's a pretty straight forward system. And works better than in years past, where people were having to stand for 4 hours to get into the comedy.
Be sure to wander/check out the visual arts galleries. Last year they blew me away...some really great curated stuff...hoping this year will also be good.
If she's looking to save money, she should pack a bagged lunch and dinner. There are some decent food vendors, but it can sure add up quickly.
This is the first year that the mainstage is in Key Arena, so that will be strange.
Bring a jacket or hoodie, as it may be chilly by the evening, and some sunscreen. Bring a refillable water bottle, as it may be 80* during the day.