From https://www.imigrasi.go.id/wna/daftar-visa-indonesia/B1 - free translation
B1 Tourist Visit Visa
...The B1 Visa is a Visa on Arrival ... for a single entry ... with an initial stay of a maximum of 30 days, counted from the date of arrival. This Stay Permit can be extended once...
Type of Visa B1 - 30 Days
1. Stay
Up to 30 days (extendable for another 30 days) ...
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So from the above information I suggest :
1) Check the exact day the extension was issued (this because it can be extended before the first 30 days finish) ;
2) Including the day the extension was issued , she has 30 permitted stay days . So the day 1 is the day when the VOA's 'Visit Stay Permit' extension was issued . Not after the day 30 , she should leave .
If she leaves on day 31 , she will probably have to pay 1 day overstay fine .
In the old days you had to count it yourself. Now it actually says on the visa. The evoa. And the B1. So no mistake. Usually. Lol
