There are several factors here.
Learning a second instrument is always quicker and easier, in the initial stages at least, than learning the first one. So much knowledge and skill regarding rhythm, note reading etc is directly transferrable, that the initial learning on the second instrument often flies by. All you are actually doing is applying a different instrumental technique to that same knowledge and skill.
Also, learning any single-line instrument is quicker in the initial stages than the piano, because you don't have the cognitive burden of conceiving and reproducing more than one line of music at a time to deal with.
When someone learns a single line instrument first then takes up the piano, these factors cancel each other out to some extent. They will probably make quicker progress with the piano than someone taking it up from scratch, OTOH they often become frustrated and don't stick at the piano because it take so much longer to start playing fluently than they are used to.
HOWEVER, when it's the other way around, and they learn the piano first then a single line instrument, the two factors COMPOUND each other, and the second instrument is often amazingly easy. I've had advanced piano students take up wind instruments so they could play in bands and get to grade 8 in literally 3 or 4 years. It can be a whole other world of quick and easy progress.
Sometimes this is not quite so true of stringed instruments because tone and intonation are so challenging. However my understanding is that these are less of an issue on the double bass than the violin.
The double bass is a cool instrument, there are not that many people that do it and it's applicable to jazz as well as classical music. It also has the same tuning and fingering as the bass guitar, so after learning it she will be able to pick up a bass guitar virtually instantly and play in bands if she wants. It's great that her piano and theory background is helpful and she's making quick progress. Enjoy!