An airline probably won’t let her board without a valid passport, so she’s best getting a new passport. She’s going to need a visa if she transits anywhere through Europe as her visa has run out you say in the comments. There are some African countries that are visa free, eVisa or visa on arrival she could transit through. Unfortunately there are no direct flights to Kenya, they will start from JFK on the 28th October 2018, see here.
Visa requirements for Kenyans
An airline can, at their discretion carry you back home with an expired passport; Even if you need to transit, as long as you are “airside” and not entering another country.
If she did manage to get back to Kenya on her expired passport she has to prove to the border officer there that she is a Kenyan citizen. Which an expired passport does prove but she will probably be questioned until they are satisfied she is a Kenyan citizen. They have to admit their own citizens, they just need proof she is one.
Emergency Certificates to travel can be issued, but I’m not sure if her i20 running out classes as an emergency, she’s had 8 years to renew her passport an should of definitely looked into doing it in the passed year if there was a chance she would have to go back to Kenya.