Well, my first thought would be to Not Multitask While Photoshopping. Photoshop wants every erg of power the computer can give, and will take it if it can. it's not always THAT bad, but it can be. Otherwise, there's good old reboot - some people's uptime is silly-long, and then they realize that there's enough memory holes in ANY os that it's time to garbage-collect by restarting.
Really, though, if it seems to be a recurring issue, there's some underlying problem that means she might want to do something like a clean reinstall of the OS. Otherwise, there's a "reply hazy, ask again later" situation - Mac issues like you describe tend to have a few things going on at once that could be any number of issues. (And, of course, the issue of what she's running on what hardware could have something to do with it as well.)
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Really, though, if it seems to be a recurring issue, there's some underlying problem that means she might want to do something like a clean reinstall of the OS. Otherwise, there's a "reply hazy, ask again later" situation - Mac issues like you describe tend to have a few things going on at once that could be any number of issues. (And, of course, the issue of what she's running on what hardware could have something to do with it as well.)
Hope that's SOME help, anyways.