Retriever trials and hunt tests will definitely stretch your equipment. I have trained a couple of labs and worked at hunt testing and tried photographing some of it. First, since these tests rarely allow anyone in the running area, it means your boyfriend will be located fairly close to the line and the dog will be running dead away from him for 50 or more yards. In hunt testing, with the long retrieve being 100 yards, a 400mm will still be a bit short for getting tight shots of the pick up. On the reverse, when you are on the line, your BF will probably be pushed into a standard or wide angle focal length approach if you want over all shots of you marking the dog. BUT, on entry, he may need to quickly hit about a 200mm length to pull the dog in close for the initial hit. As you can see, there is no one lens that works perfectly. I would personally start to think about what you want from the photos. Is the dog just running away from you for a deep fetch what you want? Personally, I liked the shots at the line with me interacting with the dog (my wife was my photog) and if possible, the send both on land and in water. The long pickups became a bit boring after a few shots of a dog picking up a duck. That said, before buying a 70-300, I would pick up a 1.4 teleconverter. It will not quite hit the 300 mm but at 280mm, it will be close enough that you can do some comparison of focal length and the fstop will be the same at the long end. It will NOT be quite as versatile of course since the 70-300 covers more range but it would be a cheaper way to see if you want that much range. If you find that you DO want the whole range of the 70-300, then go for it.
Something else to consider, It is nice for sure to get shots in competition but I found it easier to help my wife set up shots during practice. I could set her up on the far sides of ponds and shoot back across so that the fronts of my face and the dogs were visible. This also allowed good shots of the dogs face when entering the water. Usually that can't be done in a competition since the running field is off limits to spectators. Just my thoughts.