Hi @CaseyBorne88
Her original 2 year green card and original extension letter (I-797), in addition to valid passport is all she needs to leave and re-enter the US.
Of course, if any countries besides her home country and US require visa for travel - she should get one.
She cannot stay outside of the US for 6 months or more at a time - this will result in her breaking residency. If she needs to stay outside of US for 6 months but less than 2 years, she can apply for I-131.
For naturalization purposes, if she plans to apply for citizenship under 3 year rule based on marriage to US citizen, she would have spent 18 months out of 3 years in the US. The calculations are all based on Residence Since date on her card.