Extending or outstaying tourist visa for US citizen

Hello can I ask you a question about visas?
I have an American friend who wants to come out to Bolivia in August. If she gets a tourist visa for 90 days that will take her to end of October. I believe she could overstay her visa by paying when she exits 20bs/ day. Does anyone have experience of this - what is the attitude of the immigration officals when this happens? Is there any hassle?
She then wants to return to Bolivia mid January buying a new tourist visa to come in for another 90 days. Would she be able to do this, considering that she overstayed the previous year? Would they take this into account or is it seen as a new year, clean slate?
I would very much appreciate if you could share your thoughts and experiences.
Many thanks
Emma