Duolingo gives you a solid foundation in vocabulary and grammar, but your colleague has a point: it's not enough if you want to reach fluency. The app is good for training recognition and memorization, but it lacks the real-time conversation part, where you have to think fast and react. With three months you probably have the fundamentals, but to go from "I understand something" to "I can hold a conversation," you need speaking practice.
The ideal thing is to combine Duolingo with something that forces you to speak. You can look for language exchanges online (there are free platforms), conversation groups, or even a private tutor over video call a couple times a week. It doesn't necessarily have to be an expensive academy; sometimes half an hour a week with someone who actually speaks English speeds you up way more than months of the app alone.
If you stick with just Duolingo, you're going to hit a ceiling pretty fast. The app is excellent as a complement, but real listening, pronunciation, and speaking fluency require interaction with people. You could try something free first before paying for lessons, just to see if the online conversation method works for you.