If your client disputes and ACH payment and their bank accepts the request the funds will be withdrawn from your account and the dispute will automatically get marked as lost. It's not possible to submit evidence for disputed ACH payments.
Disputes on ACH payments are fundamentally different than those on credit card payments. If a customer’s bank accepts the request to return the funds for a disputed charge, Stripe immediately removes the funds from your Stripe account. Unlike credit card disputes, you can’t contest ACH reversals. You must contact your customer to resolve the situation. As a note, the ACH network allows 60 days for consumers to contest a debit.