Hello Clownchick 22,
The Nudibranch picture is of a
Hypselodoris bullockii
Chromodoris bullokii is a synonym
Your LFS Guy was partially right, They are
VERY HARD TO KEEP
He was wrong in stating that simply having a more mature aquarium is all that is needed. These Nudibranchs are exclusive sponge feeders. Often they will be specific to one or a few types of sponge making their care even more impossible. Unfortunately, buying one and hoping to have enough sponge in your system will not work. Even if you were lucky enough to have the correct sponge, the Nudibranchs feeding would quickly decimate the sponge far faster than it could regenerate. Maybe in the future alternate feeding methods will be developed but I don't think this will be anytime soon.
As to Robzilla's mention of powerhead tragedy, this is common for Nudibranchs. They have little weight and when bothered by fish they can curl up and be at the mercy of your tank currents.
Having something survive for a few weeks is not considered success.
Many fish and inverts can take weeks or even months to starve to death.
This will be the outcome of your friends nudibranch. It will start to shrink and eventually disappear.
There are only a very few Nudibranchs that can be kept successfully in a captive aquarium and even then they still have special requirements.
You SHOULD NOT BUY these animals. It is irresponsible and your purchase supports the actions of the store that should know better than to import them and offer them for sale.
Would you buy a Clownfish that was guaranteed to die within 4-6 weeks? Of course not, well this is no different.
If you must have a Nudibranch I suggest
Elysia crispata The Lettuce leaf Nudibranch
It feeds on film algae. It is prone to demise in filter intakes as well.
For further reading:
http://www.wetwebmedia.com/nudibran.htm