Well, you can buy your own sofa but she has no obligation to take the existing one away, so you could get left with two. This is really one of the downsides of renting, certainly with a furnished rent, you dont get to choose the furniture.
I think youre right, talk to her, tell her your concerns, that you want to keep the house nice etc etc. Hopefully she'll she that as a positive thing and that keeping good tenants might be better than the benefit of a free second hand sofa.
We're lucky that we can store furniture that tenants dont want away from the properties but most landlords wont be able to do this.
If you get landed with the secondhand sofa, do make sure that it's fire retardant - the labels should still be on the sofa. Older sofas might not be and so shouldnt be used by a decent landlord.