"She is seeing."
Translation:वह देख रही है।
I believe a better translation is, "That woman is watching." Morvan82 is correct as well. There are a few instances where "is seeing" works: "Are you seeing this?" (Incredulous statement, for example, you see something happening that is really bizarre or somebody doing something stupid, and you say this to you friend.) Or, "She is seeing this to the end." (Denoting resolve to complete a task that is difficult or uncomfortable.) Or, "She is seeing if she can get us a free ticket." But, "She is seeing" by itself is not used.
Please don't call it the feminine squiggle... It's called the ई की मात्रा (ē kī matra) which is represented like this ी if there is no vowel or consonant for it to accompany .It changes the pronounciation of the word by adding a 'ē' sound when it is used. I understand why you call it the feminine squiggle because its mostly used for feminine words , but please refrain from doing so .
Coming to your question, if you add (lets use ी instead of feminine squiggle) ी, the word would become देखी , and though its a feminine version, its also a past form and the word would mean the feminine version of 'saw'. You can however use देखती (feminine, present tense), like this : " वह देखती है।" which would translate into a more grammatically correct English sentence (as morvan82 said) to : "She sees."
Hope the explanation is helpful (and correct)!