No advisable to start selling cafe without work permit. Ilegal and can be caught by immigration and disturb by police unless you are willingly to pay coffee money.
Remind her to inform airport immigration that she want to cancel work permit.come in as tourist visa. If not she has to fly again if follow work permit 90days stamp.
Any country she can go as long out from thailand and when enter back she must inform airport immigration that she want to cancel work permit and ask for tourist stamp on to her passport.
Once work permit cancel , she must leave the country and enter back as tourist visa. Join new company only can get work permit. If she dont leave the country there will be overstay charges. Thb 500 . From my experience as this happen to me