My 82 Father In Law Is On Blood Thinners. He

My 82 father in law is on blood thinners. He is pretty frail, still mobile, but walks with a cane/walker and moves very slowly. Yesterday, he complained of pain in his upper left arm/shoulder. It felt like there was a small to medium knot where he described the pain. This evening his wife discovered a very large blood bruise encompassing the area of the pain and throughout his upper arm? Is this a reason to go to the emergency room? They are on their way, but just checking to see if this is a serious condition.