Speaking of concentration,
If you're concentrating on a spell (i.e. Delayed Blast Fireball) & you cast another conc. spell, does the first spell end *immediately* upon casting, or does end once the second spell's effects activate – lingering until you begin concentrating?
— Slayer Kast (@SlayerKast) January 10, 2019
“When the spell ends, either because your concentration is broken or because you decide to end it, the bead blossoms with a low roar into an explosion of flame that spreads around corners.”
That’s part of the spell description.
Specifically for “Delayed Blast Fireball” it goes off still.
But that’s because you automatically end the concentration.
I’d personally remind them that they are concentrating already. But, if they go with it, the spell ends 1st.