|Honestly, with twins washing every other day may not be possible if she uses cloth full time. When my twins were little I'd wash every day and easily had a full load. Figure that newborns go through 10-12 diapers a day, she'd have 40-48 diapers by the end of the second day. That many wouldn't likely all fit in her washer (on average you can fit about 24 diapers) nor would they likely fit in a diaper pail (even with the very large pails she'd likely need at least a second pail). No matter how many diapers she had, she'd have enough dirties to wash after just one day anyway. More would allow her a little bit more leeway in terms of when exactly she washed (and how long it took her to transfer them to the dryer) but if she waited two days she'd have two loads to wash (or close to it).
She could get away with 24 diapers if she were very diligent about washing them as soon as she was down to about 4 diapers (wash 20, keep 4 clean for changes during the washing) and drying them immediatly in the dryer (there'd be no time for line drying at all). Having 30-40 would give her more time to get the washing done or perhaps give her enough time to line dry. Getting the convenience package of 36 would give her a good package savings and she could always add a couple more if she wanted later on. If she has a tight budget and figures she could be diligent about washing most of the time she could always get a dozen prefolds as back-ups just in case.
For covers, more is better with twins unless she really needs to save $$ and doesn't mind hand-washing. I'd say 6-8 of the smaller, quickly outgrown sizes and then 10-12 of the later sizes that are worn longer.
Any of the diapers would be good, not just the One-Size. Consider her tolerance for bulk, would bulkiness bother her enough to delay using the diapers till the babies were older? Also consider their size. Were they born premature so they'd be smaller than an average 1 month old singleton? Or conversely are they large babies and likely to have already almost outgrown the usefullness of the x-small Sandy's? Or even if they're large that could indicate that even the smalls would only fit 6-8 months (instead of the 12-16 months some thinner babies can wear them). Consider also her budget and how she feels about having to buy more diapers down the line.
Also let her know that Mother-Ease gives a 10% discount to parents of twins (can't remember the exact requirements but I think you need to buy at least one full-time package).