Jennifer G. answered 03/23/19
BA in Chinese Language & Culture, lived & worked in China for 12 years
You can indicate that this particular "下次" is in the past by putting a "了” after the verb. Example:
他第一次去基加利发现了那儿没有埃塞俄比亚的餐厅, 于是他下次去了就决定开这个餐厅。I am not sure where the conversation is taking place (in Ethiopia? in China? in Rwanda?), but that will determine whether or not you use the verb "回”. Also because " 回” means "to go back", you don't really even need to use "下次" in that sentence. Here's what I mean: So he went the first time and discovered something, and then he left Kigali, right? So "when he returned" (回) implies that that is the next time he went.
You are more informed on the details of how many times he went, and where he is currently (ie: still in Kigali now?), so you be the judge of exactly how you say it. Your friend's phrase (他再次回基加利时) literally means: "when he once again returned to Kigali". That may be correct, depending on how many times he's "returned" to that place. Has he already left Kigali and returned to Kigali once, and then left again?... and this is his second time going back? If so then "再次回" could work. "He has returned once again"
This is actually kinda tricky because you not only have to choose the correct tense, but there is also the variable of location/ direction involved. Excellent question!
Hope that helped! ;-)