Should I leave a bad review for my walker who didn't show for 2 walks while I was away?

Since you already contacted the walker, the next step would be to contact Rover support. For walks booked on Rover, walkers have to complete a Rover card. It's my understanding that this requires the GPS tracking of the walker's smart phone to document arrival & departure times of each visit as well as other pertinent details along with photos. So even if the walker deleted photos, Rover should know if there is evidence to support the visits. I'd encourage you to provide as much of a review as you can based solely on your experience (not allowing other people's reviews to influence yours). It sounds like she did not send you photo/text updates of each visit and to share what your friend who stayed there reported.

For future needs, I'd suggest meeting walkers well in advance, select your best match(es), and book their services before you are away, which will help in deciding who you may want next time. During the meet & greet, that's the time to exchange expectations about how many visits, what approximate times, length of each visit, any particulars about the walk and other item the dog needs, as well as request a photo/text update of each visit. Some clients place a camera inside near the entryway or where the dogs stay, which allows you to not just see the walker, but all people coming&going in your home and if you're lucky maybe your dog.

To directly answer your question. YES you should write a poor review for the sitter who did not perform the service as agreed upon.