Husband’s ex did a runner, now CSA wants money from him

My husband has 3 children by his first partner who paid for 4months then did a runner she owes an historic debt of 80+ thousand his he had a disabled child with his second partner who also has not paid.

He has paid for the agency to chase payments and they cant locate her with all that is available to the government this person cannot be found yet today he has been sent a letter asking him to pay £25.71 a week for child he has a residence order since 2009.

Need help and info for ombudsman asap please the csa have had the details for both absent parents yet have taken so long to get the case sorted they ended up moving away this department has so many resources available but they never use then it is a total shambles