https://www.gov.uk/get-a-dna-test/if-youre-getting-a-test-to-use-in-court
Thought I'd check. If she claims he's the father then the csa will be in contact whatever.
Then when he says "it wasn't me" they'll order DNA test, which costs £250 but it's definitely court approved and still cheaper...
She needs to prove it's his, not vice versa. Unless of course he's already put himself on the birth certificate.
If she's as bad as you imply just keep away until the lawyers hand you an approved test.