That sucks about your loss. I can understand your feelings and how it would remind you of "good old days with your buddy." However, like you said there is a certain level of addiction to RC. I know that I've certainly been bitten hard by the "RC Bug." Who knows...you may be able to take up one of your old birds and be reminded of the fun that you and your father in law had. Ain't nothing wrong with that at all.
If your planes and radios still work with the fm format...there's probably nothing wrong with that. There are many websites that offer new batteries for not alot of cost.
For example:
HobbyKing RC Plane, Car, Heli, Boat and MultiRotor Hobby Store
Shop for RC Hobby Parts and Upgrades - Digital Servos, Brushless Motors, ESC, LiPo Batteries/Chargers for RC Helicopters, RC Planes, RC Cars, Robots
Home of the MicroJet V3, Super MegaJet V2, SuperSonic V2, MonsterJet Brushless ParkJet Motors, Depron Foam, Dynam RC, ESC, LiPo Batteries, Metal Gear Servos
There are many many other sites, but those three are the ones that I visit frequently for parts. If for some reason your fm setup doesn't work, I can't imagine that it would be too difficult to pick up a new 2.4gHz transmitter and receiver (e.i. Spektrum AR600 6-Channel Sport DSMX Receiver and Spektrum DX6i transmitter). Or, if you are looking to get flying without spending alot of cash, maybe the
Turnigy 9X 9Ch Transmitter w/ Module & 8ch Receiver (Mode 2) (v2 Firmware). It's a $54 transmitter that comes with a receiver and quite a few people really like this radio. There are so many choices these days.
So anyway...I'm sorry to hear about your loss but who knows....you could be out at the flying field some time this year and you could be having a great time while your father in law is looking down at you with a smile.
...and I gotta say...you are gonna really enjoy this site. I've been around here for 56 months and I gotta say...there are alot of really knowledgeable and helpful people here.
Good luck buddy!!!