He's said £550 which feels quite high for a £142k mortgage. I'm also confused by the mortgage/house value.
2003 - Buy House - Take out mortgage - £142k
2008 - Split up. Having paid £22k between them (450/month)
At some point he starts paying interest only at £550/month which is more than the actual repayment mortgage
House hasn't appreciated in 15 years.
So if he and the ex were only paying interest only and he has only been paying interest only since she left. Does that not mean when they sell, they will probably owe about 17k if the current house evaluation is correct at around 125k? Minus deposit they might come out slightly ahead I guess?