I think 'Road to Riverdale' is the better choice because it takes an inspiration from the TV show to create new stories from the same vein of Riverdale, and is a great introduction to the Archie comic series, as described here.
There is nothing wrong with the other Riverdale graphic novel you mentioned. That one is more of a direct translation from the CW series, so it may be slightly repetitive if she has already seen the show up until now.
This is the one I am referring to, though it could also be nice to add to her collection too.
Full disclosure, I have not personally read either of these titles. I was just interested in finding graphic novels as a fan myself, and this was the research I found.
I think 'R2R' is a better choice from the two because it elevates the fandom with new stories with the same characters.
Maybe another fan who has read them can chime in.
Sorry I couldn't be more helpful, but I wish you the best of luck.