People mean different things by “file taxes with [their spouse]”. Some mean “should I file as married?” to which the answer is always “Yes, you have to show that you're married” (which I know that you know, but sometimes others don't know 🙂 ). Other times they mean “Should I file as married filing jointly or as married filing separately?” which is more complicated, but based on just the limited information shared here, I'd guess that they'd come out better as MFJ than as MFS, cause his tax rate would be a lot lower as MFJ presuming that he's made more than her, but you'd need a lot more information than that to know for absolutely certain that they're better as MFJ. Sometimes they mean “should I let my spouse have my dependency exemption or should I let my parent/sister/friend/someone else”, and then again it would require more information, but probably they need to have the exemptions on one spouse or the other or on a joint return, again would need more details to know exactly. Like I knew someone once that his child married right after college, so the year that his child married, he was eligible to claim his child as a dependent because his child has been a full-time student for the majority of the year and his child had lived with him the majority of the year and some other various factors, so even though the young couple was married, the father was eligible to claim the dependent. But, that's a unique situation.
So….my point is, her question could really be asking a few different things, with different answers. But in most situations, if she's made less than $5000, and presuming he's made more and is the one supporting the household, probably MFJ (which is probably what she means by “filing with her husband”) is the best plan. However, it's not required, at least not to my knowledge – I believe she could do MFS instead if she wanted to, or if there's extenuating circumstances (like he owes back taxes, or they're having trouble and she fears separation/divorce but isn't saying anything yet, etc.), but it may result in a higher tax expense for their household as a whole.