If they're having without prejudice discussions, presuming these are fruitful, he will be asked to sign a settlement agreement. It is a legal requirement he receives advice on the contents of the agreement, therefore in 99% of cases, the employer makes a contribution to the employee's legal fees in order for them to obtain that advice. Usually £500 + VAT, especially if employer is a law firm.
Get him to make an appointment with an employment lawyer, they can help steer him in the negotiations. They could do that on or off the record (ie off the record, he would be communicating with his employer but have advice from the solicitor in the background).
Poor performance is not a gross misconduct issue.
Whatever the reason for termination (barring gross misconduct) he will need to either be given 6 months notice to work, or have it as garden leave, or being paid 6 months salary in lieu of notice. This payment will always be taxable. He will also receive any accrued but untaken holiday.
On top of this, I would expect him to receive at least some form of additional ex gratia payments. That is the nature of settlement agreements. The employer is trying to avoid any claims of unfair dismissal. By signing a settlement agreement, your DH agrees to give up any and all claims he may have against the employer, in return for benefits. He is already entitled to the notice and holiday by law - so why would he sign without an ex gratita payment? Unless of course there is something he isn't telling you. I've known of clients on the hook for gross misconduct by sexually harassing colleagues who have kept it from their families! I'm hoping this isn't the case for you.
Go and see a lawyer. It will be worth it. I could give you decent advice in an hour. And you could end up with a lot more money (maximum award for unfair dismissal in the tribunal is 12 months salary or around £90k, whichever is lower. You won't get that much with a settlement agreement, but you could comfortably get 3-6 months salary on top of notice and holiday, depending on the circumstances. More if discrimination is an issue). Ex gratia payments are tax free up to £30k. Notice and holiday always taxable. Don't forget about the value of benefits, especially if taking PILON. Restrictive covenants are a negotiating point.