The coolant that was lost must have had to go somewhere, it should be sealed system and use very little coolant, providing there is no issues and your daughter is not doing millions of miles a week…
Renault Twingo would say to me a fairly small engine and therefore probably a reasonably small cooling system and if she has added 2 litres that is probably around 1/3 to 1/2 of the cooling system capacity. When the AA refilled did they fully bleed the system as if not this may explain the reason your daughter has needed to add some more coolant as there may have been an airlock which has made its way to the expansion tank, however an airlock would probably have meant reduced or poor heater quality, usually the first thing a woman notices.
If the garage has checked and said the cooling system is fine then I see no reason to disbelieve them at the moment. Keep an eye for any puddles on the floor or excess steam from the exhaust. You can tell if its steam as it will dissipate in the air quickly whereas smoke will hang around for longer.
I would suggest that the faulty coil has nothing to do with the head gasket, they’re not connected in any real way (apart from the mechanic would have probably needed to remove to do the head gasket) and the ignition system and coolant systems are about as separate as you can get.
To summarise, keep an eye on the coolant level, engine oil level (this might go up if the coolant is making its way to the sump) and see if you’re getting constant steam from the exhaust (you will get some as the ambient temp drops and the engine warms up).
HTH