A work permit has nothing to do with it. (And she was likely given permission FOR that gig -- not to work freely in the states -- especially with unemployment the way it is.)
This has to do with Equity rules, not the government's. Equity has control over whether a show can hire a non equity, non citizen for parts. There are rules stating things like unique talents and the like. MY understanding was that in many cases, you had to convince them that there wasn't an Equity member capable of the same job. There are differences when shows transfer intact (or mostly intact) and I'm sure exceptions are made all the time.
Remember, their job is protect their members.
I understand that the rules going overseas (and particularly London) are not as difficult to overcome as are Equity's.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.