Just how long can my girl

Could she be molting? Or egg bound?  I have had besties lose their buddies by preditors and they have not displayed mourning but they have been off laying eggs or slowing.  I used to get nearly a dozen eggs a day but stress can cause them to slow in laying.....you can add some apple cidar vinagar to her water, give her a bath, or use some diatematious earth to rule out mites or other internal reasons to slow egg production. Birds will not necessarily display an illness.  Sometimes you can get an idea if they are all puffy feathers but usually it is in a birds best interest and survival not to display sickness so they mask it well.  You can try seperating her but that can cause pecking order issues later.  I would try to rule out the main reasons and go from there. 

Caroline

What happened to her hatch mate? If it was a traumatic experience she's probably stressed/in shock and possibly mourning.

I'm so sorry.  I am a firm believer that all animals are capable of mourning.  There are lots of other types of birds that display mourning, such as and especially  owls, who will mourn till they die if their mate dies.

That is sad about the loss of one of the barred rocks - I'm so sorry.  

It sounds like you have worked to rule out various health issues.  I do know that stress and changes can interrupt laying for a while.  Last summer several of my hens stopped laying after a a bad thunder storm for more than a week.  I think the loss of a hatch mate could interrupt laying for a while even though she is not acting much different.