There are two separate issues: 1) the documentary requirements for a returning resident, and 2) not abandoning residency.
The documentary requirements for a returning resident to enter the US is pretty clear. A green card (I-551) or equivalent is valid for entry after an absence of less than one year. A Re-entry Permit is valid for entry during the validity of the Re-entry Permit, which is issued for two years. An SB-1 returning resident visa is also valid for entry during its validity. She clearly meets the documentary requirements for entry.
However, to come back as a permanent resident she needs to have not abandoned residency, and that is a subjective determination that looks at many factors, not just length of time outside the US. Other things that might be considered include the pattern of her trips, whether she filed US tax returns, whether she maintained access to a home in the US that she intended to return to, whether she had family in the US that she intended to return to, whether her reason for being outside the US had a definite end date or was open-ended, whether she was prevented from returning earlier, etc.
Meeting the documentary requirements of entry does not mean that she hasn't abandoned residence. If someone repeatedly leaves the US for under one year and comes to the US for a few days and then repeats, without a permanent home in the US, they would almost certainly be determined to have abandoned residence. No length of absence, no matter how short, can absolutely guarantee that the person won't be found to have abandoned residence. Even having a Re-entry Permit doesn't mean that she hasn't abandoned residence (having a Re-entry Permit prevents a finding of abandonment due to the length of time outside the US; but it does not preclude a finding of abandonment due to other factors).
Conversely, not meeting the documentary requirements does not mean she cannot return to the US. If she returns with a green card after an absence of more than one year, the officer at entry (or the immigration judge in removal proceedings) still has the discretionary power to let her in anyway if they feels she hasn't abandoned residence.
So what really matters is whether she has abandoned residence. And unfortunately there is never a clear answer to that question because it looks at many factors and is subjective. I would say that based on what you said, she is likely okay, since she did stay in the US for a few months before leaving, so it's not an obvious case of someone hardly staying in the US at all; and it's the first time she has been away, so they are usually more lenient. She should carry evidence of her ties to the US just in case she is asked about it.
