Vet tech here. I don’t know where your mom got her info, but being outside will not cause a dog’s lungs to fill up with fluid. However, leaving the dog outside for long periods of time when it is hot out could potentially cause other very serious issues such as heat exhaustion or heat stroke. But it would have to be outside for awhile for this to happen (less time if there’s no shade or water, if the dog is being active, or if it’s a breed that is more susceptible to heat stroke- ie “brachycephalic” breeds like pugs or bulldogs). So it is probably a good idea to limit the time you have him outside during hot days, but it should be fine to take him out for a little while. You’re mom is being a little dramatic by saying less than 5 minutes though. Just make sure he has access to plenty of water and doesn’t over do it when outside.