This is different from a late payer
A customer who is 45 days late will probably pay. A customer who has filed is under an automatic stay: you cannot call them for payment, cannot pursue collection, cannot repossess goods, and any payment they made you shortly before filing can in some cases be clawed back as a preference. The invoice has become a claim in a court process with deadlines, and unsecured trade creditors are near the back of the line.
Owners who have been through it say the same thing: decide on day one that the money is a loss, file the paperwork so that any recovery is a bonus, and spend your energy on the business that is still here.
The paperwork, quickly
File a proof of claim with the invoices attached before the bar date the court sets. If your claim is not listed in the debtor's schedules, or is listed as disputed, you must file or you are out. If you delivered goods in the twenty days before the filing, that portion may qualify for priority treatment ahead of general unsecured claims — it is worth a call to a bankruptcy lawyer for an hour, because it can be the only part you ever see.
And stop shipping on open terms. If the customer is reorganizing rather than liquidating and wants to keep buying, it is cash in advance from now on, with the court's permission where needed.
Funding the hole
The $50,000 was going to pay something: this month's payroll, a supplier, the rent. Those bills have not moved. A working capital loan or a cash advance on your remaining deposits, sized to the bills that invoice was covering, funds in one to three business days. Send the bankruptcy notice with the statements; a funder reading a named customer's filing next to a specific shortfall is reading a one-time hit, not a declining business.
Amounts run half to one and a half times a month's deposits. If the lost customer was a large share of your revenue, the deposits going forward are smaller and the offer will reflect it — which is the honest signal that the business needs to be resized to the customers it still has, not just bridged.
The change that stops the next one
A single customer large enough that its bankruptcy threatens yours is the risk, and the fix is limits: a credit limit per customer, deposits on large orders, shorter terms for anyone over a set share of revenue, and a look at public records before extending terms to a new large account. A line of credit is the backstop for the year the limits are not enough.