You have received some good replies regarding the electrical symptoms.
There are at least two problems. A low battery low due to lack of use, well, proper use. Starting the engine is just half of proper use a battery gets. After each start ideally the engine should be run long enough, the car driven long enough the battery gets recharged. If the car has sat a while since last used the battery not only needs to have the power it gave up when it started the engine but the power it has lost over time.
Even with a car that is used several times a week and driven for 30 miles at a time I have observed it can take a good portion of that 30 minutes (15 minutes or more depneding upon temperature, electrical loads like lights, A/C) before the running engine and alternator have recharged the battery.
When parked the lowest power state is to of course leave nothing plugged in like a cell phone charger even if a phone is not attached and lock the car. This puts the car's electrical system in it lowest power start sooner.
The other problem is the smell of antifreeze. There's a coolant leak. This needs to be found and fixed. In the meantime you should avoid running the engine any. It doesn't read like your mechanic friend even bothered about the possibilty of a coolant leak.
The leak can be from a number of places. Water pump. Hose. Radiator. Radiator cap. Coolant tank
With the car on a lift the source of the leak should not be hard to spot. If it is with the coolant level topped up -- distiled water can be used -- and with the engine running at idle and hot with the car on a lift the leak should then show up.
Unless you have a shop nearby that is higly regarded as a place to take a Boxster and other Porsches you will probably have to make arrangements to get the car flat bedded to the dealer for a professional tech's diagnosis.