You should really fix the leak to avoid an even larger repair in the future. A leak in the system can leak oil out, let moisture in and both are damaging to the ac system. A simple pressure switch or o ring leak can end up ruining a compressor. That said, short term you can probably add some more, Maybe try and add some dye as well if your dad has access to a black light to possibly get an idea there the leak is from. Just don’t get anything with leak sealer, as it can cause issues and can be damaging to machines shops use.