The phrase around here is that the meds are scary until you see them work. Then they look like a basket of kittens.
My son was on Azathioprine, the sister drug to 6-MP, fox six months. No side effects and one pill a day for him. Unfortunately, it just wasn't strong enough for him so we had to move on. Our GI told us he has kids who've been on it for 10+ years and he only sees them once a year at their annual check ups. So it does really work for some.
She'll be monitored very closely while on 6-MP with labs. It does take awhile (months) for it to build up in the system and work so you may be looking at pred or EN to get her in a good place before the 6-MP can take over and do its thing.
Do you have a prescription for Zofran? It's for nausea and can take the edge off on tough days.
Hope she sees some relief soon!