will these assets be considered as income by the IRS, even though the
account money predates her becoming a US person?
Even if there was no income, is it possible that the person still had
to pay other taxes related to foreign assets (i.e. does the IRS tax
assets, not only income?
There are fines related to failure to disclose foreign bank accounts which are quite draconian. The fines are high out of concerns about money laundering and terrorism funding, without a legislative, IRS or judicial recognition that these issues can arise in far less nefarious circumstances.
But the fines are not truly taxes. They are fines for failure to file an information tax return or make a disclosure that is required by statute.
If the disclosure had been made in a timely manner, there would have been no actual tax due and it is not illegal to have the accounts, so long as they are disclosed.
Resolving an irregularity of this kind is quite tricky, can go very badly if done incorrectly (e.g. hundreds of thousands of dollars of civil tax fines or worse and possible impairment of immigration status), and calls for specialist international tax administration counsel.
I've encountered a case like this in my own practice and referred it out to specialist counsel rather than handling myself, even though I regularly handle less demanding international tax questions in my practice. This is a "brain surgery"/"rocket science" class difficulty problem as far as lawyer expertise requirements are concerned.