Insufficient information is often worse than no information, and it does make me sad to see how many stereotypes and wrong information is out there about feng shui, thus people missing on the opportunity to really change their lives with feng shui.
You mentioned the flowers were given to you by a special someone and you would like to keep them. Now, tell me, how can this possibly be bad feng shui? Ancient Feng Shui masters had a very beautiful saying - "Trust what your heart is thinking."
Ask your heart what it is thinking about your dried flower. If you have beautiful, healing energy connected to it, of course keep it, but do treat it in a special way. When placed on an altar, or dried in a special way, it becomes your very own feng shui symbol of something very dear to you, thus it will keep sending good energy as long as you take good care of it.
People have been drying plants and flowers since time immemorial for various healing purposes, was that bad feng shui? Lavender is a great feng shui flower that we use in dried form so often, is lavender bad feng shui? Of course, not!
The feng shui challenge with a dried flower is to keep its energy fresh and keep it special, as most of the time people keep it as a special memory. It is also important to let it go when you feel you do not need it anymore, and there is no specific timeframe here, it maybe one month for some and years for others. We all have different timeframes in dealing with the energy of powerful emotions and feelings.
If you decide to keep your dried flower, have a special place for it and do not get it lost in the dust of many items that are not needed, thus creating clutter.