I am a travel agent and I can assure you my clients do call United and modify their own bookings - not often - but it can and does happen - especially on day of departure and late at night or on weekends.
That said, if I saw a modification, I would never void a ticket, which by the way can only be done within 24 hours after the sale.
Now I do know all of my clients and don't take new ones without referrals from existing ones but it is very strange that even
an OTA would void a ticket if they saw a record modified.
I am confused that if United changed the record why the e-ticket didn't change to EXCH (exchange) status which would have made it impossible for the agency to change the status to VOID. I guess they could have revalidated it which is a more simple process where the agency can retain control but rarely done any longer.
I would have been nice if the gate would have let her go and follow up with agency accounting later and perhaps issued a debit memo to the agency or whatever was necessary to get her there.