If you want literally only one ski that will even get used on a Mammoth "deep day", you might keep an eye out for last year's Rossi Voodoo BC110 or the S3 in a 159cm. (same ski different topsheet)...both are 98mm underfoot, early rise tip and tail design, and probably enough float for any conditions for her size. These are however definitely soft-snow biased, and will only be "OK" on hard conditions. My wife is a little bigger and skis the BC110 in a 166 which is 105mm underfoot, same type of design and we're regulars at Mammoth...she loves them for any soft snow condition, deep or not. There's a woman who wrote up a review of the S3 last year in the review forum...
Also, Blizzard "Crush" but I doubt you can find a place to demo that in Mammoth.
only thing about her situation is that she could wants one ski that will be ok in any condition at Mammoth, and in a heavy snow year like last year (and some say this year), that means being prepared for that big dump, which isn't likely going to yield a ski that excels in the conditions you say she likes the most. SJ's idea of the Koa 84 could split the difference, but on new snow days, she might have more fun on S3/Crush type of ski, which by most accounts do fine on-piste in a place like Mammoth; and if she is like my wife, her terrain preferences may change quite a bit after trying a ski like that (she's gone from "70/30" to "30/70" on/off-piste preference since acquiring her BC110's)
FYI, my wife's "quiver" this season will probably be those two skis: Koa 84 159cm. + Rossi BC110 166cm.
She's going to start off seeing if she can get away with just the Rossis and add the Koa if she misses that more on-piste friendly ski.