If a novice wants to do more than 3x/week, is it detrimental to his progress?
Possibly, yes. The main factor is not whether 6x/week hurts, but that 3x/week works. Nothing is as proven in the amateur sports field as 3x/week training plans, be it in the space of weight training, running, or basically everything.
If so, why?
Doing a physical activity once per week is usually too little to have noticeable gains (which are important for a newbie, for motivation - why do it at all if it serves little perceived benefits). Twice per week is great for things like martial art training, where it's not only about the physical aspects, but where there's also a big mental part (i.e., learning a lot of new terms and techniques and so on).
Three per week is magical because it's the highest number that can be distributed over 7 days in a way that gives you a rest day inbetween each session.
Rest days are important because all the growth (muscles, endurance etc.) happens in the resting period, not during exercise. The exercises damage your muscles and other systems; in the rest periods the body overcompensates when repairing (by building a little more "stuff" in your physical systems, making you stronger).
There are also other factors at work. The central nervous system is often brought up together with overtraining - while it may seem that your muscles can take it, at some point all the other little bits and pieces of your body may fall apart if you do high intensity training daily, even though you are spacing individual muscle regions such that they get enough breaks.
Going a bit more into the mental side, newbies often start out hugely motivated. During the first days and weeks, success is immediate and super pronounced. Soon, this tends to taper off a bit, you get used to all the new activity, it becomes routine. At this point, often real life rears its ugly head - changes in the family, at work etc., and it may become harder to actually go for 6 days a week. While this may be alleviated with high motivation (at the beginning) and general discipline, it can get hard. 3 days a week is usually quite manageable.
And of course, is it wise to start with 6x/week?
I would start with 3. There are tons of proven, well-working, training plans. Something like Stronglift 5x5 (all relevant info is free, so don't take this as advertisement; some of the texts are a little opinionated but the 5 exercises employed are timeless standards) is tuned for exactly the newbie who wants to get results soon, with lots of experience on how the plan works for both well-trained and fresh people.