The advice is clear. Dwarf hamsters are notorious for falling out and these two sound as though they are already bickering. The owner really needs to be prepared to separate her hamsters at the drop of a hat as the one being `bullied` could end up dead. That`s the end scenario. If you don`t see her or her hamsters very often, then all you can do is give her the advice from this forum, but she can obviously come on herself and read the thread.
Puting it simply, buy a spare storage box with a lid from Poundstretcher or one of the other shops/supermarkets for around £10. This can be used as a Bin cage as long as it`s meshed on the top, or better still, a second hand cage like a Ferplast Mini Duna can be found locally. These can be picked up cheaply on Gumtree and other such websites. Having a spare cage handy is essential when keeping a pair of dwarf hamsters because 99% of the time, they start fighting.
Being her first hamsters, did she know this before she got them? x