It's not too late at all. There is a mom on here that I know started Doman Dots at 2.5 years and her child was doing 2 & 3 step equations and the whole deal!
I think it might have been one of the mom's with a blog that has "Kindergarten" in the title? I might be getting them confused. Doman says learning is easier at age one versus age 2, easier at age 2 versus age 3, and so forth until the window closes around age 6. So she has about 6 REALLY GREAT months and then 3 more REALLY GOOD years to teach!
Also, if all goes well she is going to get a a lot more immediate feedback from her child and validation that he is "really learning" versus teaching a 4 month old and waiting a year for results. Hopefully, s/he will be able to vocalize his knowledge a lot sooner, encouraging her to keep it up!
Go tell her to register on the forums and join us personally in this thread so we can cheer her on.