If I understand your situation correctly -
Your husband inherited a 50% share in a property from his mother.
The property was valued at €400,000 for inheritance tax purposes.
Your husband has lived in the property since the inheritance (and before that - but not relevant for capital gains tax as he only acquired the interest in the property on the death of his mother).
The property has sold for €500,000 realising a gain of €100,000.
Your husband's share of the gain for tax purposes is 50% - €50,000.
Are there are any reliefs for this gain? There is:
This is an explanation of what relief on cgt is available for a Principal Private Residence (PPR)
Your husband should be able to avail of Principal Private Residence Relief against his share of the gain and therefore should have no chargeable gain for capital gains tax.