Last I checked she wasn't very sneaky so she'd probably get found and attacked.
Actually, I think Rider
is pretty sneaky (not to mention that she can turn into spirit form to gain access). I don't think a normal person would see her if she didn't want to be seen, certainly.
Plus, you seem to have a false idea of the reason I'm asking. I am not expecting her to get away with it, but what I'm not sure about is the exact method by which she would fail. I doubt it is actually as simple as "she'll be found and attacked", she was certainly sneaky enough to get into Shirou's room and have sex with him (or steal prana, at least) without being detected
when Saber was in the next room, and I imagine that Forest has plenty of defences that might prevent her even getting that far (but, equally, Rider has high MR, so magical damage will likely not affect her much if at-all).