Client.ts

export class Client {
  doTheThing(message: string, count: number): boolean { /* ... */ }
}

index.d.ts

declare function doTheThing(message: string, count: number): boolean

index.ts

export function doTheThing(...args Parameters<Client['doTheThing']>): ReturnType<Client['doTheThing']> {
  const client = new Client(defaultClientOptions)
  return client.doTheThing(...args)
}