Is there a time limit attached to a claim for recovering financial investment in a property after a couple have split up?

My son and his partner purchased a house in 2011 with the help of a ‘gifted deposit’ from his partners aunt. They have one child who now lives with the mother.

The property was mortgaged and registered in my sons sole name as his partner had a poor credit file. When they split in 2013 she stayed in the property with the child for 6 months paying all bills and mortgage. My son moved in with me and his child stayes a few nights each week. After 6 months his partner decided to move out of the property and back with her mother and my son moved back into the house paying all bills. He lived there until March 2016 when he decided to rent out the property. This is still the case. 

Last year the aunt who gifted the deposit tried to claim it back but legal advice sought at the time confirmed she can’t do that and a letter was sent to her solicitor with a copy of her signed document confirming the gift.

Now my sons ex has sent a solicitors letter claiming she is entitled to recover her own investment in the property which is stated as:-

• A deposit
• Insurance payments
• Legal expenses for the purchase
• Mortgage payments

it is 5 years since the split, so is this a valid claim?