This is the sentence in question:
Never would he wish to injure the feelings of his schoolmate of so long ago, no matter how ridiculous/ridiculously he was acting, or how strange his beliefs.
The difficulty seems to arise over figuring out if the verb phrase is was acting, or if the verb is was, which would make acting ridiculous a gerund--or not? Is this the key to figuring out the word form?
Does the author chose the word form in order to tell the reader when the acting was happening in time, or is one form always called for in this arrangement by some solid rule? If so, what IS this arrangement?