Oh, Teri... I'm sorry to see this. I know you're considering hip replacement, but have you thought about Stem Cell therapy? They are doing amazing things for hip dysplasia, arthritis, liver disease and a few other things. I heard about it a few months ago and thought it would be something to consider for Thorins arthritis, but the cancer took him. I googled and found a youtube video that's quite inspiring:
http://www.youtube.com/watch?v=O8ZUKNE69tg
There are many articles and owner accounts/testimonials out there. This looks to be pretty promising. It's still invasive, but nothing like hip replacement, and no side effects after the treatment because they use the animals own cells. It's not as costly as hip replacement either. Good luck with Riley, and hugs to you both