Yes, that is definitely a head spline, and yes, you can expect the bow to lose value/price compared to a not broken stick. There is some disagreement on what that should be among bow makers/dealers though. Some believe it should be sold for as little as 10-20% of its unbroken value, some believe it should lose only 20-30%, and plenty believe somewhere in the middle. From a buying/selling perspective, it would likely be relatively fair to both you and the seller if you were looking at 50% of Prochownik's current price. If they are asking significantly more or less than that, you have to decide if you want to go that route.
How long has the bow had the spline for? Typically if it has been in place for a while, it should remain that way. They do occasionally fail, which largely depends on the adhesive used. If you do get a chance to see the bow in person, run a finger over the spline. A good repair is one that matches the head so there is no gap or bump when you encounter the spline, and if it is sticking out or you can see/feel a gap particularly at the bottom of the spline closest to the beak, that could indicate that it has slipped. From the photos it looks good, but you can tell a lot more with the bow in hand. There was a time when splines were done by cutting all the way through the head so the spline went right into the throat and you could see it behind the head. While that practice has largely stopped, it is still an acceptable repair provided the lines are clean and you can't see/feel any gap. It also should not look like the head has been dramatically reshaped or the line down the front of the bow has been altered, which may be difficult to determine if you are not very familiar with Prochownik's work.
As far as playing is concerned, it should play exactly the same as it did before it was broken. Since the repair is not in the playing length of the stick you do not have to worry about inconsistencies in the feel between the repaired and original stick, and the weight of the new material and adhesive should be the same or so marginally different that it is not noticeable. There are some that will tell you the sound will suffer because the head doesn't vibrate the same with a spline but again that is more in your own head than anything else, and if you are evaluating the tone of the bow in its current condition, you can make up your mind as to whether you like it or not.
My advice is to see it in person before buying or to arrange a trial if possible to have a really good look at the repair. If the repair seems solid, the bow feels and sounds good to you, and the price is right, go for it. If you are looking at the investment value of the bow, or not sure if you are going to try French for a few months, hate it, and want to sell the bow again, then be aware that it isn't going to be the easiest sell and you might end of having to price it even lower than what you are buying it for.