CSA and lottery win

[Guest]

Long story short.....a man I know has won 950k on lottery. 14 yrs ago he found out he was the father to a 6 month old boy after a brief fling. Mother had her boyfriends name on birth certificate and asked biological father and his family to take a step back so her boyfriend could raise child as his own. The biological father was quite young and foolishly agreed as did his family reluctantly. When child was 5 biological father contacted mother asking if he could have any kind of contact with child mother refused and said child believed her boyfriend was his daddy and it would be an upheaval and unfair. Child is now almost 15 and has no idea that mums boyfriend is not his biological father and here’s where things change......biological father is happily married with 2 children and has won 950k on lottery child’s mother has found out and has now decided she wants a lump sum and is contacting CSA. My understanding is that CSA can only use biological fathers earnings. £600k already been spent in new family home cars holidays etc does she have any claim in his winnings?? He has put aside money in a trust fund for his son for when he reaches 21 but won’t tell the child’s mother this as he thinks she’s just seeing ££££ signs and isn’t acting in best interests of child. He has done an online calculation for CSA and is now paying her £100 more per month than CSA would tell him to do despite having no PR not being named on birth certificate and never having any contact with his son. I’ve advised he get legal advice but it’s a tricky one he wants to do what’s morally right for his son despite the circumstances.