Who is right?
Duolingo says that: mi amigo se fue de mi casa Translates to: My friend left my house but all the translation sites say it is: My friend went to my house.
Olimar: "irse a" is possible. I could, for example, say "me voy a la cama" which could be translated to "I'm going to bed" or less naturally but more directly to "I'm leaving for bed". I could also say "me voy a pagarle" which would mean that you are going to pay him, but also that you are leaving your current location to do so.
"irse" is often difficult to translate satisfactorily to English since no equivalent exists.
Duolingo is correct: "irse" (the reflexive form of "ir") means "to leave." Nuances like this one are a major part of the difficulty in learning a language and in translation, specifically. Some things do not translate directly or literally, and computers, in particular, are especially bad at understanding these fine distinctions or using context to infer meaning.
Also, I do not think that "Mi amigo se fue a mi casa" would mean "My friend went to my house." I can't think of a time you would use "a" with "irse." It's just sort of awkward. That would be more like "My friend left to my house." When you see "irse" think of "salir."
Gracias! I've always found the reflexive verbs difficult to translate directly, in general. Honestly, I usually don't even try to translate them into English to understand them. I usually just think about them in Spanish and try to understand their meaning within that context. This situation and others like it, I think, highlight the difficulty with learning language via translation into one's native language. Not everything works so easily.
Don't worry. You will be soon. It takes time and practice. I studied Spanish for 4.5 years combined between high school and college, but I haven't used it in over 5 years, so it's super rusty for me. But the more I practice, the more it's coming back. And I'm learning new things I never knew before through the lessons and conversations like this one. This community of learners helping each other through questions like this one is, I think, one of the coolest pieces of Duolingo and is collaborative learning at work. Best of luck to you as you push ahead in your Spanish study efforts!