Ok - the horn that sounds that infernal beep beep beep ... is not the same horn that sounds when you press the horn on the steering wheel - so, you can try disconnecting it.
Now, I say try because on some of these Fords - when that horn is disconnected ( its just one wire to it that you pull off )... sometimes that will stop the car starting.... but - its worth a try... that would be the easiest way - and certainly meantime - you can do that say when the car is left for a time or overnight - to stop it sounding off... Too - you should be able to start the car - then pull the wire off the horn, that shouldn't stop the engine...and she can then drive it.
This horn in question is found under the bonnet - either at the back end of the engine bay -in the corner area - or in the "chanber" below the windscreen - just pull up the rubber trim - and then then black plastic trim- and you can easily see / get at it -
Now - as to a route cause - yes you do get issues with the light switches on the doors - but the main culprit on these is either one of the door locks -(usually the drivers door ) --the "switch " for the alarm goes faulty...or, when you open the tailgate - there on the base where the tailgate closes - you will see small square brass plates - and they tend to get all gammy -
Give them a real good clean up - there are about 4 / 5 of them - Best to use a tool such as a screwdriver or similar - to "scratch" them up....to make sure they are fully clean...
Then look to the tailgate - you will on the bottom of it - spring loaded pins - each of these contact with these contact brass plates when closed - Just make sure the ends of the pins are cleaned. C
Check they are all stuck out at the same level.... sometimes they get grimy inside meaning one or more isn't coming out / going in / working properly and hence, they dont contact the plates properly when the tailgate closes.
Any of this will set the alarm horn off...
It sounds a lot but it takes two mins really -
Hopefully that will be the cure - but - if not... tell her not to lock the car with the drivers door - but use the passenger door... ( Im assuming here it has central locking )... If so - if its a faulty switch ( which is inside the door on the door lock barrel ) - then using the pass door to lock the car / ( and she must remember to use the pass door then to unlock it to.... )... that will stop the alarm sounding off.
These door switches were a common fault -so I would suggest that she does that first - test the theory that its a fault door switch - and try that as above. If the alarm never sounds off - you know for sure thats the fault . ( unless she has a faulty passenger door switch too.... which can happen but would be somewhat unfortunate! )
So work through as above .