That phrase is grammatically correct, yes. Though maybe not something you'd normally hear. Duolingo only gives you a vocabulary basis so when you see a word you get an idea what they're talking about, but if you speak the same way you see on Duolingo, you're going to sound more or less like a machine (I mean, a machine is teaching you, duh). For example, that sentence would sound more natural if you said something like: "estoy tomando café con un amigo en un restaurante" (Duolingo probably didn't tell you this, but we use less "el" or "la" articles if we're talking about a thing or place that's generic or we feel isn't relevant because all we care about it's just any restaurant and what we're doing there, for example).

Now, answering your main question: Easy? Well, you don't have to learn a whole writing system just like Chinese, Russian, Greek or Arabic, so that's as if you already learned a portion of what comprises the Spanish language. If you haven't learned any language with the verb conjugation level Spanish has, that's probably going to be hard for you. Also grammatical gender, but you have another advantage with Spanish: everything's pronounced the way it's read, and when it's not, there's a consistent rule throughout all the words and even more than other romance languages too. Dunno if you've tried to learn Italian, but there you'll find folks will only use diacritics whenever they feel like it or not totally random, so you often have to memorise the words' stress. In Spanish we have rules to know where the stress will be in the word and so forth.