Gifted Deposit - Parent payback

My friend and her partner have recently split up after he cheated twice with various women - they are not married but cohabiting in a house with a mortgage.


They have lived in the house for a year and his parents gifted him £20,000 deposit to buy the house and do what he wishes with the money.


Since the split in January, he hasn't lived in the property and for the past 2 months hasn't paid towards the mortgage.


His solicitors have now sent my friend a letter asking for the sale of the property and for £20,000 to be returned to his parents once sold.


My question is ...


Is my friend liable to return £20,000 to his parents?
The gift was in his name rather than hers but I was under the impression that on a joint mortgage, they haven't signed a declaration of trust so the sale of the property would be shared equally.


Any help is greatly appreciated.
Thanks.