Tourist visa possible deportation??

My friend (currently out of US) sponsered his sister-in-law to visit him in the US with the assurance that she came here just for visiting the US. (It would have been difficult for his brother to sponsor his wife.) By the way my friends and his brother live in different states.

She came to the US a few weeks back, momentarily visited my brothers place and is now with her husband (in different states). Now she is thinking of working and my friend is distressed as this may cause problems for him (as he sponsored her). Is this a serious matter? and Are there any preventative action thing he should be actively doing? Any suggestion in this matter would be appreciated.

Thank you