This isn't making any sense at all.
First of all the ex would have to agree to her name being removed from the mortgage;
The lender would need to be satisfied that you and your partner have a sufficient income to pay the mortgage. They will take your payments to your mortgage in to accoutn;
It will be near impossible to remortgage in any case with �30000 negative equity;
Why would you want his name on your mortgage? If he is divorcing his ex ALL of his assets may be taken into account - that could include any properties mortgaged in his name;
If he does put his name on your mortgage your home will be affected if he is bankrupt or is in any other serious financial trouble - the house will be as much his as it yours.
Let us assume your name is on his mortgage and his name is on yours. You now both jointly own two homes. If either of you get into financial difficulty either or both properties would be at risk -as there is no equity in his house, your home would most certainly be at risk.
If there is a lot of equity in your home, his lender would be delighted to put you on the mortgage of his house. If it goes wrong, he will recover the monies by putting a charge on your house.
To be quite frank I think you would be mad to consider either taking on his �30000 negative equity and giving him a right to your equity.