Max. amount for visa on arrival?

Some countries get 90 days (tourist visa). Not many. If you are working you are illegal. You should get the proper visa to work in Malaysia.

Its all based on nationality and some have to apply for visa in advance before leaving country.

Consult the proper channels

http://www.imi.gov.my/index.php/en/main-services/visa

I am asking because a friend of mine is travelling SouthEast Asia, and she plans to enter Malaysia several times. It is not about a visa for work.

Depends on nationality

http://www.imi.gov.my/index.php/en/main-services/visa

EvaMarbach, Visa on Arrival facility is granted to those nationaility that require Visa to enter Malaysia i.e. India, Pakistan, etc. Over time, this facility has been reduced or revoked altogether.

Certain nationalities can enter Malaysia without Visa. With or without Visa, on entering, the immigration will stamp a Social Visit Pass for a specific duration in your passport to indicate permitted length of stay.

Can you share the nationality in question?

Generally, people of the Western countries are less likely to be questioned even with entering Malaysia multiple times. Africans and Asians are more prone to questioning when entering.