Yes, they will migrate looking for food (blood).
Sandy areas can harbor native flea populations. We once went to look at a rural property and had fleas jumping up our legs (ahhh!) of course we ran away.
When we had dogs with neighbors who didn't treat theirs nearby, the ONLY product that worked was the Seresto collars (4.5% flumethrin and 10% imidacloprid). From reading the reviews and other forums, there are tons of people (especially in the south) with flea populations that are immune or resistant to the older flea medications.
For saving an elderly lady from a flea infestation (the only thing they could be eating is her!) I wouldn't bother with the natural solutions, but find somewhere for her to stay for a weekend and truly knock it out. Get an exterminator in if you have to. If you can't manage, call social services. It's likely you don't want to make more stress for her, but fleas can bring deadly diseases so you may have to choose between her stress level and leaving her life in danger.
If you want to try one more DIY first, get 3-4 of those seresto collars which have a repelling effect, chop them into bits, and scatter them starting in her (bedding free) bed and around her bedroom, leaving a clear route of exit for the fleas to vacate her room out the door and perhaps the windows too. It would take some hours for them to get out. The collars release the product when they warm up, so you would want her house to be hot. When you're sure her bedroom is clear, move the collar confetti to the other living spaces. Keep in mind it's only the repelling effect that will be useful without treated blood for them to be poisoned by. Don't let anyone elderly or young have an intact collar they might think to wear and be poisoned by. That's the only DIY I can think of that might work. The typical advice is to keep vacuuming on a regular schedule, but meanwhile they are eating this old lady alive, how awful!
Spirit
Greeter
Friend
