What is the legal case for repaying arrears?

I'm completely clueless when it comes to this stuff
so I need some good input.

My girlfriend was paying support and then went to court to get
it reduced because she was going to school.
They reduced it a little bit but now I found out
she is in arrears on 2 months of payments
(the time from when she quit her job til
the reduction hearing) because
she claims she was told that it would be
retroactive and would be waived until she finished school.
Fact is that 1) it wasn't completely waived and 2) it wouldn't have been
retro even if it had been.

But my question is how much of the arrears is she
going to have to come up with immediately?
Because basically whatever it is, that means
I will have to come up with it immediately
as she isn't working and I'm footing the bill
for all of it right now.

Will they accept an offer of an extra $50 a month
until the arrears are caught up (total arrears
are $720)? What happens if they tell her to pay
the entire amount immediately and she can't?
She literally has no money in her checking
and no income at all. And we are NOT married
or engaged or anything else (only been dating a
year and a half).

I don't see how they expect her to come up with the arrears
in a more timely fashion. But just because I think that,
I'm not suggesting that the court won't do it.
Which is why I'm posting this to begin with.

The bottom line is that she has no money and no job and
jobs aren't exactly growing on trees these
days - especially for people with no degree
and no real measurable job skills. That's the
whole reason I agreed to support her while
she was going to the hair cutting school.
I was hoping someone in here would be able to
put my mind at ease and tell me that most courts are
going to accept the payment option to make up
the arrears ($50 x 15 months = $750 total - arrears are less at $720).

Thanks and, as they used to say on Dragnet, "Just
the Facts" please, Just the Facts. I know how
important those two months are to the parents and I've already had
a talk with her about it to make sure she knows it too -
especially since its an unexpected expense that I'm now
going to have to come up with!
But I can only do so much and I'm about at my limit
between supporting her, her tuition and her kids.
Any more in arrears payments and I'm going to go
off the deep end myself.

btw: This is a great place to vent. I feel
a little better getting it out. :-)

btw #2: This whole thing really makes me wonder
if getting married is a good idea after all.
The stress of this is unbelievable and it isn't
even my divorce stuff. I can't begin to imagine how you
people get through every day for years with this
stuff.