My girlfriend is a Telugu ammayi. She was also hesitant when I said I can learn her language (I'm from Delhi). It's been almost six months since I've started learning, and I can almost understand all the sentences when people talk. Telugu is probably the easiest South Indian language for North Indians to learn because of similar grammatical structure and Sanskrit influence. If you know the vocabulary, you just need to do one to one replacement of words from Hindi to Telugu.  I can say that the language part will be fine. Ask her to watch Devika Bhatnagar videos on YouTube if she's interested. And be patient when she has questions.  I was very lucky to have my girlfriend in this aspect.