Helen,
Obviously if the house is worth £198k and "if" after selling and paying off the mortgage this leaves £48k, this would be divided between them and would give £24k each. Therefore if she can't buy him out and doesn't want to stay, then this is a reasonable offer.

However, she would need to do the calculations to see if this IS a reasonable offer. She shouldn't go by what they bought the house for (unless it was a few weeks ago), as the house prices have climbed and she would need to establish by how much and then work out how much would be left over after the mortgage was settled.

Obviously the money left over should then be divided between them equally (assuming she paid an equal contribution to the deposit as well).