Thanks for your reply.
Well, legally, if the parties ar eholding the property as joint tenants or tenants in common in equal shares, both parties are entitled to 50% of the equity upon any Sale.
Only if a separate Trust Deed had been drawn up, stating that both parties agreed that your Son should receive a larger percentage of the sale proceeds, is your Son entitled to this contribution back.
Unfortunately, in any dispute, the Courts will look no further than the Land Registry Transfer document (signed by the parties shortly before they purchased) and which stipulates how the parties are to hold the property (ie jointly or otherwise), and will not hear any other evidence that one party put in a larger deposit than the other, for example.
Your Son's Solictor should/would have asked your Son and partner at the time of their purchase as to how they would like to hold the property and completed the Land Registry Transfer document accordingly.
Therefore, your Son's fiancee is legally entitled to 50% of the proceeds, after Mortgage/Agents and Solicitors fees have been paid.
I am sorry this is not the news you were looking for, but it sets out the legal position.