Working and claiming ESA

Am looking for a bit of advice please a friend is on ESA and has been for a few years, she is desperate to get back to work and is going to try to work in a shop. Its her mates shop so is going to do about ten hours minimum wage. My question is she knows all about the permitted work and its a SDM decision whether they accept it as such but my question is if they don't accept it as permitted work, Are these the rules she can work up to sixteen hours, they then would disregard the first twenty pounds and take the rest into account. She is the customer on the claim and not the partner. And are there any links to guidance.?
She is born worrier and likes to get things spot on. She is in the support group and is hoping to go full time eventually Thanks in advance, big ask but hope someone can help