I do not know the answer (I assume dry fasting is very bad for you). Mostly wanted to say that compared to (dry) religious fasting, a water fast is a breeze! Add caffeine and it gets even easier. I’ve never tried a 5 day fast though. . .
I don’t know about specific health benefits but I find dry-fasting a lot easier (but have to remind myself to take sips of water here and there). I suppose it’s because it’s what I’m used to as a Muslim.
I will add that whilst fasting has benefits, the main benefit/ point of a Muslim’s fast is building spiritual discipline and submitting to God. The main focus of Ramadan is the reconnection of the Quran (hence why Ramadan = the Month of the Quran). It’s like the reset & prep for the rest of the year. The benefit and purpose of fasting is to help us draw nearer to righteous - how? Because obtaining from food, water and other such matters during the fast is the easiest part. The real difficulty cames from avoiding obscenity, lying, vanity, anger, backbiting, conflict, falsehood etc. The means (avoiding food, drink, intercourse etc) is just the means to the goal. It keeps you aware of yourself- because certain behaviours (non-food/drink related) wjll break your fast.
And I agree with your friends- Ramadan is easily the most enjoyable time, ‘flowy’, spirtual of the year for me. Even with the fasting, the long nights, the cooking, working shift work etc.