A teenager playing a sport when ill

[ForRealMember]

I am not sporty or competitive at all. My teen daughter is a very successful sports player/not professional level but not far off. Her dad is an amazing and does all the coaching but when she is poorly he still thinks she should train. I very much disagree and stop it but I am struggling to understand his outlook.

Your DH sounds pushy and very inappropriate. If she is ill or injured she needs to rest. She will end up resenting him a d may lose illusion with the sport.
Of course depends on the nature of the illness. Bit of a sniffle - yes train. Full on chest infection or stomach bug - of course rest.

I know the plural of anecdote isn't data, but I know of a very sad case where the (very sporty and active) son of a family friend was recovering from a virus and pushed himself to run a half marathon because he'd committed with some friends to do it. He ended up with a post viral chronic fatigue and 5 years on basically can't get out of bed for more than an hour or so. It's utterly tragic. If your DD is ill she needs to rest.