S.H.
Just ask her... tell her what you told us here. That youv'e never been to a wake, that you don't know how long you need to be there, that you want to be supportive but also want to respect her boundaries etc.
Just be open and caring and tell her.
Ask her and also ask her to be open with you, about what she needs, and what you can provide... maybe she doesn't even know herself, with the whirlwind of things going on around her and her situation.
But I think, just showing her that we are all "clueless" sometimes... can be a great comfort. It shows that we are not all perfect... but that you care for her and love her and want to be there for her.
take care,
Susan