M.L.
Yes, she needs it done now b/c the decay will impact her adult teeth coming in. Not to mention pain and blood infections that can stem from untreated cavities and tooth decay. Some people are born with very thin enamel and such, that make them more susceptible to problems like this. My kids are not like this, but my nephews are and they are always at the dentist. Has the dentist suggested sealants on their teeth? It could help.
I was the opposite though. I had plenty of dental work as a kid due to braces/retainers/overcrowding... but never had a cavity or those kinds of tooth decay problems. Once I hit my 20's I get cavities all the time, I have I think 6 filling now and two cavities that need filings. Odd, I have very good dental hygiene but keep getting cavities even though they were never a problem before.