For me, if you have so much money to start with, just invest to several brokers and not only choose 1 broker. I prefer to choose broker with regulated from FCA.
I don't understand why we should trade with multiple brokers. Can you please explain why one broker is not enough to trade with?
It is not a must actually, but this way you can practically compare trading conditions, spreads, etc.. Some brokers attract with bonuses, others with low spreads and commissions... it all differs. Thus you can choose with whom you'd prefer to stick to :)
Maybe he is not getting all of expected facilities in one broker. Otherwise, he is not sure about the transparency of his 1st broker.
Accept the loss as experience