I have used frontline and advantage for over 20 yrs without issue, but I do the spray on frontline, not the top spot. I have had maybe 10 fleas in 6 yrs at most so I rarely use anything most years. See a flea? I hit them with frontline. It has been 4 yrs but I just found a flea so I ordered a new bottle on line from amazon. I know many give monthly pills but I hate giving my dog internal meds for external issues. (Except capstar, which I do use if I pick up a rescue with fleas and give it an hour or 2 to do it's job before taking it into my house!)
As for natural, people swear by them, but study after study after study finds they do very little. Fleas carry disease. Some of the store brands are worthless, others report major issues. If they only got them at kennel, then you might luck out with one treatment but probably you need one now, one in a month, one in 2 mos and that should clear it.
Cedar is toxic and no longer recommended at all (though some put in center of bedding, cover with other material, which is probably safe). Frontline and Advantage are safe for older dogs, and they have proven low rates of any issues if used right. Additionally, they sterilze the fleas so they help stop the problem.
