My 92 y.o friend suffers with knee & ankle arthritis. she is on xeralto 15mg, will using arnica gel for pain interfere with this blood thinner ? thank

Arnica gel: If you're referring to homeopathic arnica gel, it will not interfere with your friend's blood thinner. Homeopathic arnica works in an entirely different way from typical pharmaceutical medicines. If your friend is willing, she may get better results through seeing a professional homeopath about her arthritis pain. A more individualized remedy could be prescribed that way.

Very unlikely: I have had patients use topically applied arnica with blood thinners before without problem. I think it is very unlikely that there would be a problem.