I am sorry this has happened - it sounds too late to suggest relationship counselling as she thinks it is over but it they can't agree on figures I would suggest going to family mediation.
Hd also needs to speak to a family lawyer and to check if they bought as joint tenants or as tenants in common - that may affect the outcome . Joint tenants is a type of house ownership - nothing to do with the house bring a shared equity property .
Did they sign a deed of trust or cohabitation agreement to protect the deposit ? If they did or if she agrees how they deal with the deposit then it should be straightforward to sort out.
He needs to get 3 estate agents to say what they think is the realistic sale price ( rather than marketing price ) of the property is and to look at sold prices for the properties if there are a few similar properties.
As for the deposit , the key issue is whether he gets the deposit back and they then split the remaining equity they own 50/50 - or if you calculate the deposit percentage value at the time they bought the house and he gets that percentage of thd equity ( after deducting mortgage etc ) and then they split the remaining equity 50/50.
What did they agree when they bought the house and do the calculations make much difference ?
Will he want to stay at the property if they split up as too many memories etc ? If so he may be better saying it should be sold so the costs of sale ( estate agent etc are deducted and he doesn't risk the house actually being worth less than the price they agree on to calculate the equity) .
If he wants the house will he be able to afford the mortgage and rent with it being shared ownership?
As for the contents, second hand values are low so if they can't agree on values just agree a list of contents per room that they each keep - they can then sell them if they want.
Sorry he is shell shocked - this happened to a male family member who did not see it coming either and really struggled to come to terms with thd break up while his ex was steaming ahead with getting the property transferred to her.
Finally, when he is in a position to think about other matters he needs to sever the joint tenancy ( if there is one ) make a new Will and change life insurance nominations