The trustee's will deal with the POA when her solicitor requests that the assets be transferred to a new arrangement. She will need to speak with a financial advisor to arrange a new BOB or if she is in current pensionable employment she may be able to transfer the proceeds to her current scheme provided the trustees allow this.
As with your brother he can if he so chose's to, transfer the benefits to another pension arrangement after his ex has transferred.
This is the way it is normally done as if he wanted to transfer the full value of his DB scheme, his ex's solicitors would need to be notified and his ex would have to agree to this transfer. So it is a lot cleaner to look after his transfer after the POA has been completed.