Part of the problem is that there is no official or reliable benefit calculator available wrt Universal Credit - also no helpline for info. Despite the fact it is being rolled out in October this year, it is being rushed through and there are still huge problems with how HMRC are handling it in trial areas (esp linking Real Time Pay info to DWP payments).
And CAB are absolutely swamped atm. Around here, people are queuing round the block for hours to get an initial appt to assess their needs, then facing a long wait to get an appt with advisor.
As far as your friend goes - I think being in a new, difficult and vulnerable family situation means she will be scared of the future and how she manages. It is commendable that you want to help her! but (a) you can't give her concrete helpful info because it simply doesn't exist atm and (b) it's not unheard of for people to shoot the messenger anyway - she may interpret vague warning comments from you as a sign that you are not supportive. Perhaps you are best being sympathetic, and simply being clear that UC will be linked to weekly working hours, and she'll need to check that out when she can.
Being blunt, if her children are all over 5 and have no additional needs, she won't be able to choose to work 4 hours a week and UC. She is likely to be asked in for interview, have to prove she is looking for work with more hours, and if work is found for her that is deemed suitable, she will have to take it or lose benefits. i wouldn't want to be the bearer of that news.
Mumsnet have a guide to UC which is as good as any atm. You could mention that to her.