She has a good vocabulary and she is quite smart and grasps things fast but she can't speak even one sentence without using a million ahhs and umms and fillers like "you know" or "that/ this thing" and she mixes words too.
For instance if she wanted to say " I went shopping with x and y today" she would say something like "today x and me and even y ahh umm when whats that ... shopping"
but sometimes she speaks fluently without any errors. how can I help her out?
This sounds more like a neurological problem than a speech problem per se.
If it wasn't un-PC I'd say it sounds like she needs a smack on the head - all that dithering would drive you mad!
Maybe she has no confidence is being listened to? Have you tried telling her to just think what she wants to say and say it out straight?
No, a speech therapist is more used for enunciation of words, not for bad grammar.
She's probably nervous...are you close friends? Is she shy?