Ale Luna

My husband is a conditional green card holder (through marriage). Right before Covid, we went on a four month backpacking trip through Europe. He had no issue coming back in. 

That won't be an issue. Just make sure you don't break your continuous residency (ie don't stay outside the US for 6 months or more in a row) and meet the physical presence requirement (ie spend at least half the time of the last 5 years inside the US, this rule needs to be followed throughout the whole N-400 process and not just at the time of filing). Since you haven't left the county since you became an LPR, the physical presence shouldn't be an issue for you.

Do you have a conditional two year green card or an unconditional 10 year green card? If you were married for more than 2 years when you issued your original LPR then you should have the unconditional LPR status and the 10 year green card. If you were married less than 2 years then you must file a i751 to remove the conditions on your LPR. If you don’t there will be major issues (even if USCIS issued you a 10 year card in error).
 
A green card holder who files for naturalization must in the USA more than outside the USA, the number of days inside the USA must exceed the number of days outside the USA over the three years before applying for your n400. Also, you must not be out of the USA for more than 180 days at a time (note day count not 6 months) over the 3 year period.
 
As far as keeping your LPR you can file for an reentry permit to stay out of the USA up to two years (unconditional LPR) otherwise don’t be out of USA for more than a year and naturalization will need to wait until you meet the more time in USA and the 180 day rule.

Never late to do AR-11 online. 

She married 3 months ago, so she cannot apply for citizenship now. 
As you just said, she can apply once she's been an LPR for 5 years, demonstrating continuous residence. 

To be eligible, you must have been living in marital union with your U.S. citizen spouse during the three years immediately before the date you file your application.
 
So, no, you cannot apply for naturalization now. 

I think you misunderstood her question a bit. It reads to me as she already has a green card obtained through her employer prior to meeting (and then marrying) a US citizen. OP, can you please clarify or confirm?
The more important question for you OP is, are you still living in the US ie maintaining your residency? Yes, you can apply for citizenship on your own after being a legal permanent resident for 5 years.