I've seen this happen many times and no-one seemed to care that much as far as I could see - they might lament that they wished they had time to practice more... but they can see for themselves that the other person has put in more training time and energy. And at the kyu level in particular, testing requires a certain number of 'days of training' so if you haven't got them, you can count for yourself and see you can't test for a while longer... As long as the promotions are clearly based on training hours and skill and there's no appearance of politics or anything, people seem to take it as a matter of course that someone who trains more hours a week will usually progress faster. If in doubt they can always look at the attendance records and compare number of days of training, or watch the person's test. Also people often go through phases -- someone trains 4-5 days a week, then something changes (usually they either have a new child or go to school) and cut back to once a week for a year or two, then after a few years get back up to 3, or in some cases back to 4-5...
Seriously, even if they trained for ten years for two days a week, you've never seen someone reach a real 3rd kyu level when looking at their skill?
Sure they'll make 3rd kyu... but so what? If, as I think is generally agreed upon, Shodan represents the point at which one is considered a serious beginner, that the person at that point has enough of a foundation to really start training on something with a bit more depth, not advanced yet, but at least a bit deeper... then what would the point be in taking ten years just to get to 3rd kyu? 15 years to Shodan? Unless one started Aikido at age five, at that pace, one would NEVER actually get to the point at which anything of any substance could be taught. They'd spend their entire lives learning the basic motor skills. I see no value in that at all. It's not even Aikido-lite...