There is a recent discussion about (roughly) this on FlyerTalk - happily also Avianca, from which I conclude your would-be fellow passengers need not fear your depriving them of their seats.
When you try to cancel tickets they are actually processed one at a time (you pay $50 per ticket) ... It should be absolutely not problem to cancel just one of the two tickets. Just call the Avianca call-center and say you wish to cancel a reservation, then explain you wish to cancel only one out of the two passengers.
(scibuff Feb 3, 16, 6:28 pm)
It may be worth contacting Avianca, with a good reason, to see if your return flight could be held for you regardless of a fairly standard sector convention that failing to show up for any one of a series of flights booked as a series will give rise to cancellation of all the remainder of the series.
That is, if you are considering flying out a little later than presently booked but would like to return according to your original schedule.