Second leg of a multi leg flight cancelled by airline - what are my rights / options

My daughter booked a oneway Paris -> Sao Paulo -> Bogata flight with carrier LATAM though govoyages.com. The flight is in 4 weeks.
A few days after booking she was notified by govoyages.com that the Sao Paulo -> Bogata leg has been canceled by LATAM. Govoyages just said the leg is canceled and they are pursuing reimbursement with LATAM. They didn't offer any alternative flights or any further information. There phone line suggest the reimbursement processes could be very long e.g. 6 months.
Looking at flight options to replace this leg is looking complicated.
What are our rights with regard to:
a. Cancelling the first leg with either govoyages or LATAM (and hence effectively the whole trip) so we are able to rebook a NEW Paris -> Bogata rather than scraping round for a Sao Paulo -> Bogata leg which will like cost as much as the whole trip. OR
b. Getting a replacement Sao Paulo -> Bogata leg offered by LATAM to replace the flight they canceled.
Currently we have half a flight which is pretty useless. Any advice much appreciated.
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Can I get refund for second leg of flight?
You catch the first leg of the flight, but when you land you learn the second leg is canceled. Now imagine the airline can't offer you an alternative that gets you to your original destination within a reasonable timeframe. If that is the case, you are within your rights to demand a full refund of the flight ticket.What happens if my second flight is Cancelled?
What happens now? The good news is that most airlines will rebook you for free on their next flight to your destination as long as the flight has available seats. If there are none available, get ready to put on your most courteous voice. The DOT suggests asking to have your ticket transferred to another airline.What happens if part of your flight is Cancelled?
What should I do if my flight is canceled? If your flight is canceled by the airline, you will either be accommodated on a later flight or, if you decide not to travel, you are entitled to a full refund under federal law. If the flight had multiple stops, you would be refunded for the unused portion of the flight.What rights do you have if your flight is Cancelled?
If your flight is cancelled and you choose to cancel your trip as a result, you are entitled to a refund for the unused transportation \u2013 even for non-refundable tickets. You are also entitled to a refund for any bag fee that you paid, and any extras you may have purchased, such as a seat assignment.Hundreds of flights cancelled for holiday weekend
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Ron Lach, Ron Lach, Ron Lach, Karolina Grabowska