Seresto works pretty well but I don't usually like to recommend it due a dog I had in the past having a severe reaction. It started to leave ulcer like burns and a rash around his neck under the collar. It's important to note though that an animal can have a negative reaction to any flea medication, and it's important to watch them closely after you start them on one. I just don't prefer the collars, but many people swear by Seresto so it's a bit daft for me to say oral preventatives or nothing. She can certainly give it a go if it's easier on her budget. Just check under the collar regularly as well as watching for the usual symptoms.
If ticks aren't an issue, then Trifexis would be a cheaper alternative to Heartgard and Nexgard. It prevents heartworms and fleas in one pill, but not ticks.