
Non steroidal anti inflammatories do carry risks of kidney and gastrointestinal problems. I looked into this a fair bit when my then 6 year old collie was going lame. I wouldn't use them in a dog of this age if I could avoid them, (and I did manage to sort it out with supplements) but in a dog of 13 I think you just need to weigh up the pros and cons.
I did not come across anyone who had problems in the long run if their dog didn't react poorly immediately to the drug. That's obviously not to say it doesn't happen, but the benefit of the drug to your girl may outweigh the chance of kidney damage, at 13 in a fairly big breed I would probably take that route if I felt it helped the dog, there were not other realistic options, and there was no reaction to begin with. I would also be doing regular blood tests to check organ function.
Of course you could always try time and physiotherapy. It's still early days and if she was a fit and healthy 13 year old before perhaps it's something she could bounce back from :)