As - usual the moment lawyers become involved you turn a very simple issue into a confusing mess.
1. Fundamental principle- she did not waive her 5'th amend rights because she CAN'T. A constitutional right is exactly that -a constitutional right. It is ALWAYS in force.
2. The argument is irrelevant. All asserting the 5'th says is that you won't help someone convict you. It is NOT an assertion of innocence or denial of lawbreaking.
3. It does NOT prevent anyone from asking you questions in this type of setting. She may invoke the 5'th after any question. Continued questioning would clearly establish that she is withholding evidence of wrongdoing or believes in her OWN mind that what she did could well be lawbreaking.