Buying Another Canon Lens

I wish a good photographer-friend is around to help me decide. A photographer-friend is selling her Canon EF-S 18-200 F3.5 after buying a new Canon L lens (her second one). While I am still drooling on her new Canon lens of which I cannot afford (maybe, if I win the lotto), I am thinking of buying her 18-200 but what's stopping is that I already have a Canon EF-S 55-250 and if I buy the other camera, would it be worth it? Help!

Reading the reviews is not really helping. It's confusing me more so I want somebody who had experience using it aside from the seller. Unfortunately, I cannot find one. Hopefully, there'd be somebody here who could enlighten me before I make my final decision.