See "A researcher has wrongly claimed co-authorship of my publication" in https://explore.researchgate.net/display/support/Authorship for instructions how to correct this. But unlike this help page suggests, most probably it was not the other author himself who wrongly claimed co-authorship, but ResearchGate's automatic algorithm wrongly identified another researcher as the co-author and assigned this publication to his profile. Unfortunately, this is a frequent problem in ResearchGate.