I am not questioning what Erin pays. It is reasonable and an adult can be supported for £50.00 per week for say food and share of household bills, but it wouldn't cover any rent and I think it is a good idea to charge an amount for pure 'rent'. However, if the offspring is saving perhaps to get a home of their own or for a deposit on rental, then I think £50.00 would be fair to give them a chance to save.
At least Erin contributes, a lot of parents quite happily don't charge their kids anything at all.