Almost anything can be ethical if it's done openly and honestly and nobodies contribution is misrepresented to anyone involved.
If Elizabeth makes it absolutely clear to anyone judging the thesis exactly what has come from Rose and in what context then there's no ethical breach.
There could however still be a breach of rules since the rules can still forbid this kind of "help" from third parties for a thesis.
The difference between a breach of ethics vs a breach of rules is important since it can mean the difference between a simple "fail" or rejection for including too much material from a 3rd party or in breach of some rule vs being investigated for grossly unethical behavior. One is actively attempting to mislead someone, the other is just screwing up.
If Rose were certain that Elizabeth was going to be open and honest then there would be no ethical issue for Rose, it would be up to Elizabeth to make sure that everything fit the rules of her institution but practically speaking given the context Rose should reasonably expect that perfect disclosure may not be what Elizabeth plans.
Hypothetically if Rose were to provide this "help" in a context where it seems unlikely that Elizabeth will act perfectly ethically it would be fair to also consider Rose's actions unethical and it may reflect badly on her if the situation came to light.
If Elizabeth has no problem with Rose meeting with, say, her advisor/supervisor and/or someone else senior from her institution and discussing how it will work then it may be ok.
Otherwise run a mile.