How does releasing equity within the property work? Does it mean that the house may have to be sold to release funds?

Thanks for your help.

My son owes @£35K on 6 credit cards; he also pays nearly £200 a month on a mortgage (I believe his property will currently be in negative equity). The joint income of he and his wife is @£2K a month - this is variable, though seldom more; currently she is on maternity leave and they have repayment e...