Without looking @ the code it would be hard to say how this could be done. Depending on how the theme was set-up will depend on how easy is it to make it non-responsive. The easier solution was one suggested by @ralph_m;, I recommend this solution, however, personally, I like easy solutions, which for me would be a similar theme of non-responsive nature.
Your solution also works fine, but I much prefer @ralph_m; solution. You're right, for edits use the child theme, and never the parent. I am not too sure how child themes are setup from a developers point of view, as I've not created one, I soon will however. =)