I would recommend introducing tempo work, as well as longer race pace intervals. By doing 500s and 600s at or close to race pace, you train the body to accept pushing through after the 400. it creates that same race lactate that your runner starts struggling with.
This is a workout from Brad Hudson's Little Black Book (a book FULL of workouts for just about every occasion for every type of runner)
3x600m with 6-8 minutes recovery between reps
- First rep a little slower than race pace to make the workout doable.
- Second rep right at race pace
- Third rep either under or right at race pace
This will build strength in pushing past that 400. The third 200 is typically the slowest, so this is all about getting through that comfortably.
A good workout for preparing for these fast races that also hits this sweet spot is:
1x 500/1 min recovery/300
This is an all out 500 with short rest to trigger all of that lactate. The 300 is whatever is left, teaching and conditioning the body to run fast through all of the tightening and heavy muscles.
Needless to say it's very hard, but teaches you and the runner a lot about how their body is going to handle the last half of a race.