The OP never said that they even asked the FIL for a ride, perhaps he volunteered. My mom would volunteer to take us and drop us off........and we live TWO HOURS from the airport. It's just that she would never want to leave her car parked at an airport all week and she wouldn't want us to/would assume we didn't want to, as well. She wouldn't want to drive either of our cars. We would most likely fill her van up for her to get us there and back, I suppose. We have taken her and picked her up and not asked for money to cover the cost. Since the OP never said what the arrangements were, I think it would be judgmental to just up and decide they were trying to swindle anything out of him. She never said how far they were from the airport, perhaps gas isn't that big a factor.
I guess, were we in that situation, I would look at a taxi or other car service. I really, really don't like the idea of leaving my car unattended in an airport lot. When DH flew a lot for work he drove a beat up, old van that we HOPED would get stolen LOL But our *good* cars? No!! The ideas about the water at restaurants or about cutting out one meal and eating in the room are good ones, if that is something you can do. I don't know your meal plans so it might not work for you. What about starting a mini-fund and putting all loose change and one dollar bills in it? You'd be suprised how FAST that adds up and we really don't even miss the money we are putting in there. Change and ones usually go for junk we don't really *need* anyway, like candy or cokes from the vending machines at my work when my kids come after school. Now I buy a few drinks/snacks on sale and keep them on hand to fend off their munchies. Much cheaper and, since it is part of my grocery budget, it doesn't hurt! Just an example, that scenario might well not apply to you. If you have a weekly night out or you get take out some during the week, cut that out for now and pocket the money you would have spent. Little things go a long way!