In reply to molly2202:
Summer. For the Matterhorn July > August > early September, though it can equally be out of condition during any of that time! Same for Mont Blanc but also include the second half of June. Mont Blanc is skiable in the spring, but as you gave your mate's climbing experience I assume you mean a climbing ascent. For guided ascents, Mont Blanc is on a ratio of 2:1 but the Matterhorn is 1:1.