I have to agree with Jaybird. When you are breastfeeding a young baby, expressing is not recommended (they can get confused between bottle and breast) and using formula can affect milk supply. So if breastfeeding is the choice that she has made, the mother is the only one who can feed the baby at night. But it can be really exhausting.
Definately the best offer would be for some uninterrupted sleep in the knowledge that someone else is looking after baby. If you bring dinner too, that would be a bonus.
For tips on getting the baby to sleep, I would recommend 'The Baby Whisperer,how to calm, connect and communicate with your Baby', it worked for me