From the Data Sheet: "The I2C circuitry is powered from VBUS when a supply is connected. If the VBUS supply is not connected, the I2C circuitry is powered from the battery through BAT. The battery voltage must stay above 2.5V with no input connected in order to maintain proper operation."
From what I see in the Data Sheet, the high level logic level is 1.3 V (Vih min) to 4.44 V (VBatreg) + 4.17%, which equals Vsys max. The Data Sheet recommends 10 k pull ups but you can use 2.2 k or so for faster edges.
Please follow the Data Sheet I2C SCL/SDA timing recommendations and reference the NXP I2C specification for more details. The customer can also send a schematic and scope shots of SCL/SDA RD/WR cycles for review.